diff options
| author | David Howells <dhowells@redhat.com> | 2021-11-10 14:25:03 +0100 |
|---|---|---|
| committer | David Howells <dhowells@redhat.com> | 2022-01-11 23:13:01 +0100 |
| commit | e0484344c0413e1fcd5642b77d49c7648fb194ec (patch) | |
| tree | f3d7ed889f38860476ce5269d1f3db88d915930b /Documentation/filesystems | |
| parent | ceph: add fscache writeback support (diff) | |
| download | linux-e0484344c0413e1fcd5642b77d49c7648fb194ec.tar.xz linux-e0484344c0413e1fcd5642b77d49c7648fb194ec.zip | |
fscache: Rewrite documentation
Rewrite the fscache documentation.
Changes
=======
ver #3:
- The volume coherency data is now an arbitrarily-sized blob, not a u64.
ver #2:
- Put quoting around some bits of C being referred to in the docs[1].
- Stripped the markup off the ref to the netfs lib doc[2].
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20211130175119.63d0e7aa@canb.auug.org.au/ [1]
Link: https://lore.kernel.org/r/20211130162311.105fcfa5@canb.auug.org.au/ [2]
Link: https://lore.kernel.org/r/163819672252.215744.15454333549935901588.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906986754.143852.17703291789683936950.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967193834.1823006.15991526817786159772.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021585970.640689.3162537597817521032.stgit@warthog.procyon.org.uk/ # v4
Diffstat (limited to 'Documentation/filesystems')
| -rw-r--r-- | Documentation/filesystems/caching/backend-api.rst | 850 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/cachefiles.rst | 6 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/fscache.rst | 525 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/index.rst | 4 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/netfs-api.rst | 1136 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/object.rst | 313 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/operations.rst | 210 | ||||
| -rw-r--r-- | Documentation/filesystems/netfs_library.rst | 16 |
8 files changed, 815 insertions, 2245 deletions
diff --git a/Documentation/filesystems/caching/backend-api.rst b/Documentation/filesystems/caching/backend-api.rst index 19fbf6b9aa36..be793c49a772 100644 --- a/Documentation/filesystems/caching/backend-api.rst +++ b/Documentation/filesystems/caching/backend-api.rst @@ -1,727 +1,479 @@ .. SPDX-License-Identifier: GPL-2.0 -========================== -FS-Cache Cache backend API -========================== +================= +Cache Backend API +================= The FS-Cache system provides an API by which actual caches can be supplied to FS-Cache for it to then serve out to network filesystems and other interested -parties. +parties. This API is used by:: -This API is declared in <linux/fscache-cache.h>. + #include <linux/fscache-cache.h>. -Initialising and Registering a Cache -==================================== - -To start off, a cache definition must be initialised and registered for each -cache the backend wants to make available. For instance, CacheFS does this in -the fill_super() operation on mounting. - -The cache definition (struct fscache_cache) should be initialised by calling:: - - void fscache_init_cache(struct fscache_cache *cache, - struct fscache_cache_ops *ops, - const char *idfmt, - ...); - -Where: - - * "cache" is a pointer to the cache definition; - - * "ops" is a pointer to the table of operations that the backend supports on - this cache; and - - * "idfmt" is a format and printf-style arguments for constructing a label - for the cache. - - -The cache should then be registered with FS-Cache by passing a pointer to the -previously initialised cache definition to:: - - int fscache_add_cache(struct fscache_cache *cache, - struct fscache_object *fsdef, - const char *tagname); - -Two extra arguments should also be supplied: - - * "fsdef" which should point to the object representation for the FS-Cache - master index in this cache. Netfs primary index entries will be created - here. FS-Cache keeps the caller's reference to the index object if - successful and will release it upon withdrawal of the cache. - - * "tagname" which, if given, should be a text string naming this cache. If - this is NULL, the identifier will be used instead. For CacheFS, the - identifier is set to name the underlying block device and the tag can be - supplied by mount. - -This function may return -ENOMEM if it ran out of memory or -EEXIST if the tag -is already in use. 0 will be returned on success. - - -Unregistering a Cache -===================== - -A cache can be withdrawn from the system by calling this function with a -pointer to the cache definition:: - - void fscache_withdraw_cache(struct fscache_cache *cache); - -In CacheFS's case, this is called by put_super(). - - -Security +Overview ======== -The cache methods are executed one of two contexts: - - (1) that of the userspace process that issued the netfs operation that caused - the cache method to be invoked, or - - (2) that of one of the processes in the FS-Cache thread pool. - -In either case, this may not be an appropriate context in which to access the -cache. - -The calling process's fsuid, fsgid and SELinux security identities may need to -be masqueraded for the duration of the cache driver's access to the cache. -This is left to the cache to handle; FS-Cache makes no effort in this regard. - +Interaction with the API is handled on three levels: cache, volume and data +storage, and each level has its own type of cookie object: -Control and Statistics Presentation -=================================== + ======================= ======================= + COOKIE C TYPE + ======================= ======================= + Cache cookie struct fscache_cache + Volume cookie struct fscache_volume + Data storage cookie struct fscache_cookie + ======================= ======================= -The cache may present data to the outside world through FS-Cache's interfaces -in sysfs and procfs - the former for control and the latter for statistics. +Cookies are used to provide some filesystem data to the cache, manage state and +pin the cache during access in addition to acting as reference points for the +API functions. Each cookie has a debugging ID that is included in trace points +to make it easier to correlate traces. Note, though, that debugging IDs are +simply allocated from incrementing counters and will eventually wrap. -A sysfs directory called /sys/fs/fscache/<cachetag>/ is created if CONFIG_SYSFS -is enabled. This is accessible through the kobject struct fscache_cache::kobj -and is for use by the cache as it sees fit. +The cache backend and the network filesystem can both ask for cache cookies - +and if they ask for one of the same name, they'll get the same cookie. Volume +and data cookies, however, are created at the behest of the filesystem only. -Relevant Data Structures -======================== +Cache Cookies +============= - * Index/Data file FS-Cache representation cookie:: +Caches are represented in the API by cache cookies. These are objects of +type:: - struct fscache_cookie { - struct fscache_object_def *def; - struct fscache_netfs *netfs; - void *netfs_data; - ... - }; - - The fields that might be of use to the backend describe the object - definition, the netfs definition and the netfs's data for this cookie. - The object definition contain functions supplied by the netfs for loading - and matching index entries; these are required to provide some of the - cache operations. - - - * In-cache object representation:: - - struct fscache_object { - int debug_id; - enum { - FSCACHE_OBJECT_RECYCLING, - ... - } state; - spinlock_t lock - struct fscache_cache *cache; - struct fscache_cookie *cookie; + struct fscache_cache { + void *cache_priv; + unsigned int debug_id; + char *name; ... }; - Structures of this type should be allocated by the cache backend and - passed to FS-Cache when requested by the appropriate cache operation. In - the case of CacheFS, they're embedded in CacheFS's internal object - structures. +There are a few fields that the cache backend might be interested in. The +``debug_id`` can be used in tracing to match lines referring to the same cache +and ``name`` is the name the cache was registered with. The ``cache_priv`` +member is private data provided by the cache when it is brought online. The +other fields are for internal use. - The debug_id is a simple integer that can be used in debugging messages - that refer to a particular object. In such a case it should be printed - using "OBJ%x" to be consistent with FS-Cache. - Each object contains a pointer to the cookie that represents the object it - is backing. An object should retired when put_object() is called if it is - in state FSCACHE_OBJECT_RECYCLING. The fscache_object struct should be - initialised by calling fscache_object_init(object). +Registering a Cache +=================== +When a cache backend wants to bring a cache online, it should first register +the cache name and that will get it a cache cookie. This is done with:: - * FS-Cache operation record:: + struct fscache_cache *fscache_acquire_cache(const char *name); - struct fscache_operation { - atomic_t usage; - struct fscache_object *object; - unsigned long flags; - #define FSCACHE_OP_EXCLUSIVE - void (*processor)(struct fscache_operation *op); - void (*release)(struct fscache_operation *op); - ... - }; +This will look up and potentially create a cache cookie. The cache cookie may +have already been created by a network filesystem looking for it, in which case +that cache cookie will be used. If the cache cookie is not in use by another +cache, it will be moved into the preparing state, otherwise it will return +busy. - FS-Cache has a pool of threads that it uses to give CPU time to the - various asynchronous operations that need to be done as part of driving - the cache. These are represented by the above structure. The processor - method is called to give the op CPU time, and the release method to get - rid of it when its usage count reaches 0. +If successful, the cache backend can then start setting up the cache. In the +event that the initialisation fails, the cache backend should call:: - An operation can be made exclusive upon an object by setting the - appropriate flag before enqueuing it with fscache_enqueue_operation(). If - an operation needs more processing time, it should be enqueued again. + void fscache_relinquish_cookie(struct fscache_cache *cache); +to reset and discard the cookie. - * FS-Cache retrieval operation record:: - struct fscache_retrieval { - struct fscache_operation op; - struct address_space *mapping; - struct list_head *to_do; - ... - }; +Bringing a Cache Online +======================= - A structure of this type is allocated by FS-Cache to record retrieval and - allocation requests made by the netfs. This struct is then passed to the - backend to do the operation. The backend may get extra refs to it by - calling fscache_get_retrieval() and refs may be discarded by calling - fscache_put_retrieval(). +Once the cache is set up, it can be brought online by calling:: - A retrieval operation can be used by the backend to do retrieval work. To - do this, the retrieval->op.processor method pointer should be set - appropriately by the backend and fscache_enqueue_retrieval() called to - submit it to the thread pool. CacheFiles, for example, uses this to queue - page examination when it detects PG_lock being cleared. + int fscache_add_cache(struct fscache_cache *cache, + const struct fscache_cache_ops *ops, + void *cache_priv); - The to_do field is an empty list available for the cache backend to use as - it sees fit. +This stores the cache operations table pointer and cache private data into the +cache cookie and moves the cache to the active state, thereby allowing accesses +to take place. - * FS-Cache storage operation record:: +Withdrawing a Cache From Service +================================ - struct fscache_storage { - struct fscache_operation op; - pgoff_t store_limit; - ... - }; +The cache backend can withdraw a cache from service by calling this function:: - A structure of this type is allocated by FS-Cache to record outstanding - writes to be made. FS-Cache itself enqueues this operation and invokes - the write_page() method on the object at appropriate times to effect - storage. + void fscache_withdraw_cache(struct fscache_cache *cache); +This moves the cache to the withdrawn state to prevent new cache- and +volume-level accesses from starting and then waits for outstanding cache-level +accesses to complete. -Cache Operations -================ +The cache must then go through the data storage objects it has and tell fscache +to withdraw them, calling:: -The cache backend provides FS-Cache with a table of operations that can be -performed on the denizens of the cache. These are held in a structure of type: + void fscache_withdraw_cookie(struct fscache_cookie *cookie); - :: +on the cookie that each object belongs to. This schedules the specified cookie +for withdrawal. This gets offloaded to a workqueue. The cache backend can +test for completion by calling:: - struct fscache_cache_ops + bool fscache_are_objects_withdrawn(struct fscache_cookie *cache); - * Name of cache provider [mandatory]:: +Once all the cookies are withdrawn, a cache backend can withdraw all the +volumes, calling:: - const char *name + void fscache_withdraw_volume(struct fscache_volume *volume); - This isn't strictly an operation, but should be pointed at a string naming - the backend. +to tell fscache that a volume has been withdrawn. This waits for all +outstanding accesses on the volume to complete before returning. +When the the cache is completely withdrawn, fscache should be notified by +calling:: - * Allocate a new object [mandatory]:: + void fscache_cache_relinquish(struct fscache_cache *cache); - struct fscache_object *(*alloc_object)(struct fscache_cache *cache, - struct fscache_cookie *cookie) +to clear fields in the cookie and discard the caller's ref on it. - This method is used to allocate a cache object representation to back a - cookie in a particular cache. fscache_object_init() should be called on - the object to initialise it prior to returning. - This function may also be used to parse the index key to be used for - multiple lookup calls to turn it into a more convenient form. FS-Cache - will call the lookup_complete() method to allow the cache to release the - form once lookup is complete or aborted. +Volume Cookies +============== +Within a cache, the data storage objects are organised into logical volumes. +These are represented in the API as objects of type:: - * Look up and create object [mandatory]:: + struct fscache_volume { + struct fscache_cache *cache; + void *cache_priv; + unsigned int debug_id; + char *key; + unsigned int key_hash; + ... + u8 coherency_len; + u8 coherency[]; + }; - void (*lookup_object)(struct fscache_object *object) +There are a number of fields here that are of interest to the caching backend: - This method is used to look up an object, given that the object is already - allocated and attached to the cookie. This should instantiate that object - in the cache if it can. + * ``cache`` - The parent cache cookie. - The method should call fscache_object_lookup_negative() as soon as - possible if it determines the object doesn't exist in the cache. If the - object is found to exist and the netfs indicates that it is valid then - fscache_obtained_object() should be called once the object is in a - position to have data stored in it. Similarly, fscache_obtained_object() - should also be called once a non-present object has been created. + * ``cache_priv`` - A place for the cache to stash private data. - If a lookup error occurs, fscache_object_lookup_error() should be called - to abort the lookup of that object. + * ``debug_id`` - A debugging ID for logging in tracepoints. + * ``key`` - A printable string with no '/' characters in it that represents + the index key for the volume. The key is NUL-terminated and padded out to + a multiple of 4 bytes. - * Release lookup data [mandatory]:: + * ``key_hash`` - A hash of the index key. This should work out the same, no + matter the cpu arch and endianness. - void (*lookup_complete)(struct fscache_object *object) + * ``coherency`` - A piece of coherency data that should be checked when the + volume is bound to in the cache. - This method is called to ask the cache to release any resources it was - using to perform a lookup. + * ``coherency_len`` - The amount of data in the coherency buffer. - * Increment object refcount [mandatory]:: +Data Storage Cookies +==================== - struct fscache_object *(*grab_object)(struct fscache_object *object) +A volume is a logical group of data storage objects, each of which is +represented to the network filesystem by a cookie. Cookies are represented in +the API as objects of type:: - This method is called to increment the reference count on an object. It - may fail (for instance if the cache is being withdrawn) by returning NULL. - It should return the object pointer if successful. + struct fscache_cookie { + struct fscache_volume *volume; + void *cache_priv; + unsigned long flags; + unsigned int debug_id; + unsigned int inval_counter; + loff_t object_size; + u8 advice; + u32 key_hash; + u8 key_len; + u8 aux_len; + ... + }; +The fields in the cookie that are of interest to the cache backend are: - * Lock/Unlock object [mandatory]:: + * ``volume`` - The parent volume cookie. - void (*lock_object)(struct fscache_object *object) - void (*unlock_object)(struct fscache_object *object) + * ``cache_priv`` - A place for the cache to stash private data. - These methods are used to exclusively lock an object. It must be possible - to schedule with the lock held, so a spinlock isn't sufficient. + * ``flags`` - A collection of bit flags, including: + * FSCACHE_COOKIE_NO_DATA_TO_READ - There is no data available in the + cache to be read as the cookie has been created or invalidated. - * Pin/Unpin object [optional]:: + * FSCACHE_COOKIE_NEEDS_UPDATE - The coherency data and/or object size has + been changed and needs committing. - int (*pin_object)(struct fscache_object *object) - void (*unpin_object)(struct fscache_object *object) + * FSCACHE_COOKIE_LOCAL_WRITE - The netfs's data has been modified + locally, so the cache object may be in an incoherent state with respect + to the server. - These methods are used to pin an object into the cache. Once pinned an - object cannot be reclaimed to make space. Return -ENOSPC if there's not - enough space in the cache to permit this. + * FSCACHE_COOKIE_HAVE_DATA - The backend should set this if it + successfully stores data into the cache. + * FSCACHE_COOKIE_RETIRED - The cookie was invalidated when it was + relinquished and the cached data should be discarded. - * Check coherency state of an object [mandatory]:: + * ``debug_id`` - A debugging ID for logging in tracepoints. - int (*check_consistency)(struct fscache_object *object) + * ``inval_counter`` - The number of invalidations done on the cookie. - This method is called to have the cache check the saved auxiliary data of - the object against the netfs's idea of the state. 0 should be returned - if they're consistent and -ESTALE otherwise. -ENOMEM and -ERESTARTSYS - may also be returned. + * ``advice`` - Information about how the cookie is to be used. - * Update object [mandatory]:: + * ``key_hash`` - A hash of the index key. This should work out the same, no + matter the cpu arch and endianness. - int (*update_object)(struct fscache_object *object) + * ``key_len`` - The length of the index key. - This is called to update the index entry for the specified object. The - new information should be in object->cookie->netfs_data. This can be - obtained by calling object->cookie->def->get_aux()/get_attr(). + * ``aux_len`` - The length of the coherency data buffer. +Each cookie has an index key, which may be stored inline to the cookie or +elsewhere. A pointer to this can be obtained by calling:: - * Invalidate data object [mandatory]:: + void *fscache_get_key(struct fscache_cookie *cookie); - int (*invalidate_object)(struct fscache_operation *op) +The index key is a binary blob, the storage for which is padded out to a +multiple of 4 bytes. - This is called to invalidate a data object (as pointed to by op->object). - All the data stored for this object should be discarded and an - attr_changed operation should be performed. The caller will follow up - with an object update operation. +Each cookie also has a buffer for coherency data. This may also be inline or +detached from the cookie and a pointer is obtained by calling:: - fscache_op_complete() must be called on op before returning. + void *fscache_get_aux(struct fscache_cookie *cookie); - * Discard object [mandatory]:: - void (*drop_object)(struct fscache_object *object) +Cookie Accounting +================= - This method is called to indicate that an object has been unbound from its - cookie, and that the cache should release the object's resources and - retire it if it's in state FSCACHE_OBJECT_RECYCLING. +Data storage cookies are counted and this is used to block cache withdrawal +completion until all objects have been destroyed. The following functions are +provided to the cache to deal with that:: - This method should not attempt to release any references held by the - caller. The caller will invoke the put_object() method as appropriate. + void fscache_count_object(struct fscache_cache *cache); + void fscache_uncount_object(struct fscache_cache *cache); + void fscache_wait_for_objects(struct fscache_cache *cache); +The count function records the allocation of an object in a cache and the +uncount function records its destruction. Warning: by the time the uncount +function returns, the cache may have been destroyed. - * Release object reference [mandatory]:: +The wait function can be used during the withdrawal procedure to wait for +fscache to finish withdrawing all the objects in the cache. When it completes, +there will be no remaining objects referring to the cache object or any volume +objects. - void (*put_object)(struct fscache_object *object) - This method is used to discard a reference to an object. The object may - be freed when all the references to it are released. +Cache Management API +==================== +The cache backend implements the cache management API by providing a table of +operations that fscache can use to manage various aspects of the cache. These +are held in a structure of type:: - * Synchronise a cache [mandatory]:: + struct fscache_cache_ops { + const char *name; + ... + }; - void (*sync)(struct fscache_cache *cache) +This contains a printable name for the cache backend driver plus a number of +pointers to methods to allow fscache to request management of the cache: - This is called to ask the backend to synchronise a cache with its backing - device. + * Set up a volume cookie [optional]:: + void (*acquire_volume)(struct fscache_volume *volume); - * Dissociate a cache [mandatory]:: + This method is called when a volume cookie is being created. The caller + holds a cache-level access pin to prevent the cache from going away for + the duration. This method should set up the resources to access a volume + in the cache and should not return until it has done so. - void (*dissociate_pages)(struct fscache_cache *cache) + If successful, it can set ``cache_priv`` to its own data. - This is called to ask a cache to perform any page dissociations as part of - cache withdrawal. + * Clean up volume cookie [optional]:: - * Notification that the attributes on a netfs file changed [mandatory]:: + void (*free_volume)(struct fscache_volume *volume); - int (*attr_changed)(struct fscache_object *object); + This method is called when a volume cookie is being released if + ``cache_priv`` is set. - This is called to indicate to the cache that certain attributes on a netfs - file have changed (for example the maximum size a file may reach). The - cache can read these from the netfs by calling the cookie's get_attr() - method. - The cache may use the file size information to reserve space on the cache. - It should also call fscache_set_store_limit() to indicate to FS-Cache the - highest byte it's willing to store for an object. + * Look up a cookie in the cache [mandatory]:: - This method may return -ve if an error occurred or the cache object cannot - be expanded. In such a case, the object will be withdrawn from service. + bool (*lookup_cookie)(struct fscache_cookie *cookie); - This operation is run asynchronously from FS-Cache's thread pool, and - storage and retrieval operations from the netfs are excluded during the - execution of this operation. + This method is called to look up/create the resources needed to access the + data storage for a cookie. It is called from a worker thread with a + volume-level access pin in the cache to prevent it from being withdrawn. + True should be returned if successful and false otherwise. If false is + returned, the withdraw_cookie op (see below) will be called. - * Reserve cache space for an object's data [optional]:: + If lookup fails, but the object could still be created (e.g. it hasn't + been cached before), then:: - int (*reserve_space)(struct fscache_object *object, loff_t size); + void fscache_cookie_lookup_negative( + struct fscache_cookie *cookie); - This is called to request that cache space be reserved to hold the data - for an object and the metadata used to track it. Zero size should be - taken as request to cancel a reservation. + can be called to let the network filesystem proceed and start downloading + stuff whilst the cache backend gets on with the job of creating things. - This should return 0 if successful, -ENOSPC if there isn't enough space - available, or -ENOMEM or -EIO on other errors. + If successful, ``cookie->cache_priv`` can be set. - The reservation may exceed the current size of the object, thus permitting - future expansion. If the amount of space consumed by an object would - exceed the reservation, it's permitted to refuse requests to allocate - pages, but not required. An object may be pruned down to its reservation - size if larger than that already. + * Withdraw an object without any cookie access counts held [mandatory]:: - * Request page be read from cache [mandatory]:: + void (*withdraw_cookie)(struct fscache_cookie *cookie); - int (*read_or_alloc_page)(struct fscache_retrieval *op, - struct page *page, - gfp_t gfp) + This method is called to withdraw a cookie from service. It will be + called when the cookie is relinquished by the netfs, withdrawn or culled + by the cache backend or closed after a period of non-use by fscache. - This is called to attempt to read a netfs page from the cache, or to - reserve a backing block if not. FS-Cache will have done as much checking - as it can before calling, but most of the work belongs to the backend. + The caller doesn't hold any access pins, but it is called from a + non-reentrant work item to manage races between the various ways + withdrawal can occur. - If there's no page in the cache, then -ENODATA should be returned if the - backend managed to reserve a backing block; -ENOBUFS or -ENOMEM if it - didn't. + The cookie will have the ``FSCACHE_COOKIE_RETIRED`` flag set on it if the + associated data is to be removed from the cache. - If there is suitable data in the cache, then a read operation should be - queued and 0 returned. When the read finishes, fscache_end_io() should be - called. - The fscache_mark_pages_cached() should be called for the page if any cache - metadata is retained. This will indicate to the netfs that the page needs - explicit uncaching. This operation takes a pagevec, thus allowing several - pages to be marked at once. + * Change the size of a data storage object [mandatory]:: - The retrieval record pointed to by op should be retained for each page - queued and released when I/O on the page has been formally ended. - fscache_get/put_retrieval() are available for this purpose. + void (*resize_cookie)(struct netfs_cache_resources *cres, + loff_t new_size); - The retrieval record may be used to get CPU time via the FS-Cache thread - pool. If this is desired, the op->op.processor should be set to point to - the appropriate processing routine, and fscache_enqueue_retrieval() should - be called at an appropriate point to request CPU time. For instance, the - retrieval routine could be enqueued upon the completion of a disk read. - The to_do field in the retrieval record is provided to aid in this. + This method is called to inform the cache backend of a change in size of + the netfs file due to local truncation. The cache backend should make all + of the changes it needs to make before returning as this is done under the + netfs inode mutex. - If an I/O error occurs, fscache_io_error() should be called and -ENOBUFS - returned if possible or fscache_end_io() called with a suitable error - code. + The caller holds a cookie-level access pin to prevent a race with + withdrawal and the netfs must have the cookie marked in-use to prevent + garbage collection or culling from removing any resources. - fscache_put_retrieval() should be called after a page or pages are dealt - with. This will complete the operation when all pages are dealt with. + * Invalidate a data storage object [mandatory]:: - * Request pages be read from cache [mandatory]:: + bool (*invalidate_cookie)(struct fscache_cookie *cookie); - int (*read_or_alloc_pages)(struct fscache_retrieval *op, - struct list_head *pages, - unsigned *nr_pages, - gfp_t gfp) + This is called when the network filesystem detects a third-party + modification or when an O_DIRECT write is made locally. This requests + that the cache backend should throw away all the data in the cache for + this object and start afresh. It should return true if successful and + false otherwise. - This is like the read_or_alloc_page() method, except it is handed a list - of pages instead of one page. Any pages on which a read operation is - started must be added to the page cache for the specified mapping and also - to the LRU. Such pages must also be removed from the pages list and - ``*nr_pages`` decremented per page. + On entry, new I O/operations are blocked. Once the cache is in a position + to accept I/O again, the backend should release the block by calling:: - If there was an error such as -ENOMEM, then that should be returned; else - if one or more pages couldn't be read or allocated, then -ENOBUFS should - be returned; else if one or more pages couldn't be read, then -ENODATA - should be returned. If all the pages are dispatched then 0 should be - returned. + void fscache_resume_after_invalidation(struct fscache_cookie *cookie); + If the method returns false, caching will be withdrawn for this cookie. - * Request page be allocated in the cache [mandatory]:: - int (*allocate_page)(struct fscache_retrieval *op, - struct page *page, - gfp_t gfp) + * Prepare to make local modifications to the cache [mandatory]:: - This is like the read_or_alloc_page() method, except that it shouldn't - read from the cache, even if there's data there that could be retrieved. - It should, however, set up any internal metadata required such that - the write_page() method can write to the cache. + void (*prepare_to_write)(struct fscache_cookie *cookie); - If there's no backing block available, then -ENOBUFS should be returned - (or -ENOMEM if there were other problems). If a block is successfully - allocated, then the netfs page should be marked and 0 returned. + This method is called when the network filesystem finds that it is going + to need to modify the contents of the cache due to local writes or + truncations. This gives the cache a chance to note that a cache object + may be incoherent with respect to the server and may need writing back + later. This may also cause the cached data to be scrapped on later + rebinding if not properly committed. - * Request pages be allocated in the cache [mandatory]:: + * Begin an operation for the netfs lib [mandatory]:: - int (*allocate_pages)(struct fscache_retrieval *op, - struct list_head *pages, - unsigned *nr_pages, - gfp_t gfp) + bool (*begin_operation)(struct netfs_cache_resources *cres, + enum fscache_want_state want_state); - This is an multiple page version of the allocate_page() method. pages and - nr_pages should be treated as for the read_or_alloc_pages() method. + This method is called when an I/O operation is being set up (read, write + or resize). The caller holds an access pin on the cookie and must have + marked the cookie as in-use. + If it can, the backend should attach any resources it needs to keep around + to the netfs_cache_resources object and return true. - * Request page be written to cache [mandatory]:: + If it can't complete the setup, it should return false. - int (*write_page)(struct fscache_storage *op, - struct page *page); + The want_state parameter indicates the state the caller needs the cache + object to be in and what it wants to do during the operation: - This is called to write from a page on which there was a previously - successful read_or_alloc_page() call or similar. FS-Cache filters out - pages that don't have mappings. + * ``FSCACHE_WANT_PARAMS`` - The caller just wants to access cache + object parameters; it doesn't need to do data I/O yet. - This method is called asynchronously from the FS-Cache thread pool. It is - not required to actually store anything, provided -ENODATA is then - returned to the next read of this page. + * ``FSCACHE_WANT_READ`` - The caller wants to read data. - If an error occurred, then a negative error code should be returned, - otherwise zero should be returned. FS-Cache will take appropriate action - in response to an error, such as withdrawing this object. + * ``FSCACHE_WANT_WRITE`` - The caller wants to write to or resize the + cache object. - If this method returns success then FS-Cache will inform the netfs - appropriately. + Note that there won't necessarily be anything attached to the cookie's + cache_priv yet if the cookie is still being created. - * Discard retained per-page metadata [mandatory]:: +Data I/O API +============ - void (*uncache_page)(struct fscache_object *object, struct page *page) +A cache backend provides a data I/O API by through the netfs library's ``struct +netfs_cache_ops`` attached to a ``struct netfs_cache_resources`` by the +``begin_operation`` method described above. - This is called when a netfs page is being evicted from the pagecache. The - cache backend should tear down any internal representation or tracking it - maintains for this page. +See the Documentation/filesystems/netfs_library.rst for a description. -FS-Cache Utilities -================== +Miscellaneous Functions +======================= FS-Cache provides some utilities that a cache backend may make use of: * Note occurrence of an I/O error in a cache:: - void fscache_io_error(struct fscache_cache *cache) + void fscache_io_error(struct fscache_cache *cache); - This tells FS-Cache that an I/O error occurred in the cache. After this - has been called, only resource dissociation operations (object and page - release) will be passed from the netfs to the cache backend for the - specified cache. + This tells FS-Cache that an I/O error occurred in the cache. This + prevents any new I/O from being started on the cache. This does not actually withdraw the cache. That must be done separately. + * Note cessation of caching on a cookie due to failure:: - * Invoke the retrieval I/O completion function:: - - void fscache_end_io(struct fscache_retrieval *op, struct page *page, - int error); - - This is called to note the end of an attempt to retrieve a page. The - error value should be 0 if successful and an error otherwise. - - - * Record that one or more pages being retrieved or allocated have been dealt - with:: - - void fscache_retrieval_complete(struct fscache_retrieval *op, - int n_pages); - - This is called to record the fact that one or more pages have been dealt - with and are no longer the concern of this operation. When the number of - pages remaining in the operation reaches 0, the operation will be - completed. - - - * Record operation completion:: - - void fscache_op_complete(struct fscache_operation *op); - - This is called to record the completion of an operation. This deducts - this operation from the parent object's run state, potentially permitting - one or more pending operations to start running. - - - * Set highest store limit:: - - void fscache_set_store_limit(struct fscache_object *object, - loff_t i_size); - - This sets the limit FS-Cache imposes on the highest byte it's willing to - try and store for a netfs. Any page over this limit is automatically - rejected by fscache_read_alloc_page() and co with -ENOBUFS. - - - * Mark pages as being cached:: - - void fscache_mark_pages_cached(struct fscache_retrieval *op, - struct pagevec *pagevec); - - This marks a set of pages as being cached. After this has been called, - the netfs must call fscache_uncache_page() to unmark the pages. - - - * Perform coherency check on an object:: - - enum fscache_checkaux fscache_check_aux(struct fscache_object *object, - const void *data, - uint16_t datalen); - - This asks the netfs to perform a coherency check on an object that has - just been looked up. The cookie attached to the object will determine the - netfs to use. data and datalen should specify where the auxiliary data - retrieved from the cache can be found. - - One of three values will be returned: - - FSCACHE_CHECKAUX_OKAY - The coherency data indicates the object is valid as is. - - FSCACHE_CHECKAUX_NEEDS_UPDATE - The coherency data needs updating, but otherwise the object is - valid. - - FSCACHE_CHECKAUX_OBSOLETE - The coherency data indicates that the object is obsolete and should - be discarded. - - - * Initialise a freshly allocated object:: - - void fscache_object_init(struct fscache_object *object); - - This initialises all the fields in an object representation. - - - * Indicate the destruction of an object:: - - void fscache_object_destroyed(struct fscache_cache *cache); - - This must be called to inform FS-Cache that an object that belonged to a - cache has been destroyed and deallocated. This will allow continuation - of the cache withdrawal process when it is stopped pending destruction of - all the objects. - - - * Indicate negative lookup on an object:: - - void fscache_object_lookup_negative(struct fscache_object *object); - - This is called to indicate to FS-Cache that a lookup process for an object - found a negative result. - - This changes the state of an object to permit reads pending on lookup - completion to go off and start fetching data from the netfs server as it's - known at this point that there can't be any data in the cache. - - This may be called multiple times on an object. Only the first call is - significant - all subsequent calls are ignored. - - - * Indicate an object has been obtained:: - - void fscache_obtained_object(struct fscache_object *object); - - This is called to indicate to FS-Cache that a lookup process for an object - produced a positive result, or that an object was created. This should - only be called once for any particular object. - - This changes the state of an object to indicate: - - (1) if no call to fscache_object_lookup_negative() has been made on - this object, that there may be data available, and that reads can - now go and look for it; and - - (2) that writes may now proceed against this object. - - - * Indicate that object lookup failed:: - - void fscache_object_lookup_error(struct fscache_object *object); - - This marks an object as having encountered a fatal error (usually EIO) - and causes it to move into a state whereby it will be withdrawn as soon - as possible. - - - * Indicate that a stale object was found and discarded:: - - void fscache_object_retrying_stale(struct fscache_object *object); - - This is called to indicate that the lookup procedure found an object in - the cache that the netfs decided was stale. The object has been - discarded from the cache and the lookup will be performed again. - - - * Indicate that the caching backend killed an object:: - - void fscache_object_mark_killed(struct fscache_object *object, - enum fscache_why_object_killed why); - - This is called to indicate that the cache backend preemptively killed an - object. The why parameter should be set to indicate the reason: + void fscache_caching_failed(struct fscache_cookie *cookie); - FSCACHE_OBJECT_IS_STALE - - the object was stale and needs discarding. + This notes that a the caching that was being done on a cookie failed in + some way, for instance the backing storage failed to be created or + invalidation failed and that no further I/O operations should take place + on it until the cache is reset. - FSCACHE_OBJECT_NO_SPACE - - there was insufficient cache space + * Count I/O requests:: - FSCACHE_OBJECT_WAS_RETIRED - - the object was retired when relinquished. + void fscache_count_read(void); + void fscache_count_write(void); - FSCACHE_OBJECT_WAS_CULLED - - the object was culled to make space. + These record reads and writes from/to the cache. The numbers are + displayed in /proc/fs/fscache/stats. + * Count out-of-space errors:: - * Get and release references on a retrieval record:: + void fscache_count_no_write_space(void); + void fscache_count_no_create_space(void); - void fscache_get_retrieval(struct fscache_retrieval *op); - void fscache_put_retrieval(struct fscache_retrieval *op); + These record ENOSPC errors in the cache, divided into failures of data + writes and failures of filesystem object creations (e.g. mkdir). - These two functions are used to retain a retrieval record while doing - asynchronous data retrieval and block allocation. + * Count objects culled:: + void fscache_count_culled(void); - * Enqueue a retrieval record for processing:: + This records the culling of an object. - void fscache_enqueue_retrieval(struct fscache_retrieval *op); + * Get the cookie from a set of cache resources:: - This enqueues a retrieval record for processing by the FS-Cache thread - pool. One of the threads in the pool will invoke the retrieval record's - op->op.processor callback function. This function may be called from - within the callback function. + struct fscache_cookie *fscache_cres_cookie(struct netfs_cache_resources *cres) + Pull a pointer to the cookie from the cache resources. This may return a + NULL cookie if no cookie was set. - * List of object state names:: - const char *fscache_object_states[]; +API Function Reference +====================== - For debugging purposes, this may be used to turn the state that an object - is in into a text string for display purposes. +.. kernel-doc:: include/linux/fscache-cache.h diff --git a/Documentation/filesystems/caching/cachefiles.rst b/Documentation/filesystems/caching/cachefiles.rst index e58bc1fd312a..8bf396b76359 100644 --- a/Documentation/filesystems/caching/cachefiles.rst +++ b/Documentation/filesystems/caching/cachefiles.rst @@ -1,8 +1,8 @@ .. SPDX-License-Identifier: GPL-2.0 -=============================================== -CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM -=============================================== +=================================== +Cache on Already Mounted Filesystem +=================================== .. Contents: diff --git a/Documentation/filesystems/caching/fscache.rst b/Documentation/filesystems/caching/fscache.rst index 70de86922b6a..a74d7b052dc1 100644 --- a/Documentation/filesystems/caching/fscache.rst +++ b/Documentation/filesystems/caching/fscache.rst @@ -10,25 +10,25 @@ Overview This facility is a general purpose cache for network filesystems, though it could be used for caching other things such as ISO9660 filesystems too. -FS-Cache mediates between cache backends (such as CacheFS) and network +FS-Cache mediates between cache backends (such as CacheFiles) and network filesystems:: +---------+ - | | +--------------+ - | NFS |--+ | | - | | | +-->| CacheFS | - +---------+ | +----------+ | | /dev/hda5 | - | | | | +--------------+ - +---------+ +-->| | | - | | | |--+ - | AFS |----->| FS-Cache | - | | | |--+ - +---------+ +-->| | | - | | | | +--------------+ - +---------+ | +----------+ | | | - | | | +-->| CacheFiles | - | ISOFS |--+ | /var/cache | - | | +--------------+ + | | +--------------+ + | NFS |--+ | | + | | | +-->| CacheFS | + +---------+ | +----------+ | | /dev/hda5 | + | | | | +--------------+ + +---------+ +-------------->| | | + | | +-------+ | |--+ + | AFS |----->| | | FS-Cache | + | | | netfs |-->| |--+ + +---------+ +-->| lib | | | | + | | | | | | +--------------+ + +---------+ | +-------+ +----------+ | | | + | | | +-->| CacheFiles | + | 9P |--+ | /var/cache | + | | +--------------+ +---------+ Or to look at it another way, FS-Cache is a module that provides a caching @@ -84,101 +84,62 @@ then serving the pages out of that cache rather than the netfs inode because: one-off access of a small portion of it (such as might be done with the "file" program). -It instead serves the cache out in PAGE_SIZE chunks as and when requested by -the netfs('s) using it. +It instead serves the cache out in chunks as and when requested by the netfs +using it. FS-Cache provides the following facilities: - (1) More than one cache can be used at once. Caches can be selected + * More than one cache can be used at once. Caches can be selected explicitly by use of tags. - (2) Caches can be added / removed at any time. + * Caches can be added / removed at any time, even whilst being accessed. - (3) The netfs is provided with an interface that allows either party to + * The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (2)). - (4) The interface to the netfs returns as few errors as possible, preferring + * The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious. - (5) Cookies are used to represent indices, files and other objects to the - netfs. The simplest cookie is just a NULL pointer - indicating nothing - cached there. - - (6) The netfs is allowed to propose - dynamically - any index hierarchy it - desires, though it must be aware that the index search function is - recursive, stack space is limited, and indices can only be children of - indices. - - (7) Data I/O is done direct to and from the netfs's pages. The netfs - indicates that page A is at index B of the data-file represented by cookie - C, and that it should be read or written. The cache backend may or may - not start I/O on that page, but if it does, a netfs callback will be - invoked to indicate completion. The I/O may be either synchronous or - asynchronous. - - (8) Cookies can be "retired" upon release. At this point FS-Cache will mark - them as obsolete and the index hierarchy rooted at that point will get - recycled. - - (9) The netfs provides a "match" function for index searches. In addition to - saying whether a match was made or not, this can also specify that an - entry should be updated or deleted. - -(10) As much as possible is done asynchronously. - - -FS-Cache maintains a virtual indexing tree in which all indices, files, objects -and pages are kept. Bits of this tree may actually reside in one or more -caches:: - - FSDEF - | - +------------------------------------+ - | | - NFS AFS - | | - +--------------------------+ +-----------+ - | | | | - homedir mirror afs.org redhat.com - | | | - +------------+ +---------------+ +----------+ - | | | | | | - 00001 00002 00007 00125 vol00001 vol00002 - | | | | | - +---+---+ +-----+ +---+ +------+------+ +-----+----+ - | | | | | | | | | | | | | - PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak - | | - PG0 +-------+ - | | - 00001 00003 - | - +---+---+ - | | | - PG0 PG1 PG2 - -In the example above, you can see two netfs's being backed: NFS and AFS. These -have different index hierarchies: - - * The NFS primary index contains per-server indices. Each server index is - indexed by NFS file handles to get data file objects. Each data file - objects can have an array of pages, but may also have further child - objects, such as extended attributes and directory entries. Extended - attribute objects themselves have page-array contents. - - * The AFS primary index contains per-cell indices. Each cell index contains - per-logical-volume indices. Each of volume index contains up to three - indices for the read-write, read-only and backup mirrors of those volumes. - Each of these contains vnode data file objects, each of which contains an - array of pages. - -The very top index is the FS-Cache master index in which individual netfs's -have entries. - -Any index object may reside in more than one cache, provided it only has index -children. Any index with non-index object children will be assumed to only -reside in one cache. + * There are three types of cookie: cache, volume and data file cookies. + Cache cookies represent the cache as a whole and are not normally visible + to the netfs; the netfs gets a volume cookie to represent a collection of + files (typically something that a netfs would get for a superblock); and + data file cookies are used to cache data (something that would be got for + an inode). + + * Volumes are matched using a key. This is a printable string that is used + to encode all the information that might be needed to distinguish one + superblock, say, from another. This would be a compound of things like + cell name or server address, volume name or share path. It must be a + valid pathname. + + * Cookies are matched using a key. This is a binary blob and is used to + represent the object within a volume (so the volume key need not form + part of the blob). This might include things like an inode number and + uniquifier or a file handle. + + * Cookie resources are set up and pinned by marking the cookie in-use. + This prevents the backing resources from being culled. Timed garbage + collection is employed to eliminate cookies that haven't been used for a + short while, thereby reducing resource overload. This is intended to be + used when a file is opened or closed. + + A cookie can be marked in-use multiple times simultaneously; each mark + must be unused. + + * Begin/end access functions are provided to delay cache withdrawal for the + duration of an operation and prevent structs from being freed whilst + we're looking at them. + + * Data I/O is done by asynchronous DIO to/from a buffer described by the + netfs using an iov_iter. + + * An invalidation facility is available to discard data from the cache and + to deal with I/O that's in progress that is accessing old data. + + * Cookies can be "retired" upon release, thereby causing the object to be + removed from the cache. The netfs API to FS-Cache can be found in: @@ -189,11 +150,6 @@ The cache backend API to FS-Cache can be found in: Documentation/filesystems/caching/backend-api.rst -A description of the internal representations and object state machine can be -found in: - - Documentation/filesystems/caching/object.rst - Statistical Information ======================= @@ -201,333 +157,162 @@ Statistical Information If FS-Cache is compiled with the following options enabled:: CONFIG_FSCACHE_STATS=y - CONFIG_FSCACHE_HISTOGRAM=y -then it will gather certain statistics and display them through a number of -proc files. +then it will gather certain statistics and display them through: -/proc/fs/fscache/stats ----------------------- + /proc/fs/fscache/stats - This shows counts of a number of events that can happen in FS-Cache: +This shows counts of a number of events that can happen in FS-Cache: +--------------+-------+-------------------------------------------------------+ |CLASS |EVENT |MEANING | +==============+=======+=======================================================+ -|Cookies |idx=N |Number of index cookies allocated | -+ +-------+-------------------------------------------------------+ -| |dat=N |Number of data storage cookies allocated | +|Cookies |n=N |Number of data storage cookies allocated | + +-------+-------------------------------------------------------+ -| |spc=N |Number of special cookies allocated | -+--------------+-------+-------------------------------------------------------+ -|Objects |alc=N |Number of objects allocated | -+ +-------+-------------------------------------------------------+ -| |nal=N |Number of object allocation failures | +| |v=N |Number of volume index cookies allocated | + +-------+-------------------------------------------------------+ -| |avl=N |Number of objects that reached the available state | -+ +-------+-------------------------------------------------------+ -| |ded=N |Number of objects that reached the dead state | -+--------------+-------+-------------------------------------------------------+ -|ChkAux |non=N |Number of objects that didn't have a coherency check | +| |vcol=N |Number of volume index key collisions | + +-------+-------------------------------------------------------+ -| |ok=N |Number of objects that passed a coherency check | -+ +-------+-------------------------------------------------------+ -| |upd=N |Number of objects that needed a coherency data update | -+ +-------+-------------------------------------------------------+ -| |obs=N |Number of objects that were declared obsolete | -+--------------+-------+-------------------------------------------------------+ -|Pages |mrk=N |Number of pages marked as being cached | -| |unc=N |Number of uncache page requests seen | +| |voom=N |Number of OOM events when allocating volume cookies | +--------------+-------+-------------------------------------------------------+ |Acquire |n=N |Number of acquire cookie requests seen | + +-------+-------------------------------------------------------+ -| |nul=N |Number of acq reqs given a NULL parent | -+ +-------+-------------------------------------------------------+ -| |noc=N |Number of acq reqs rejected due to no cache available | -+ +-------+-------------------------------------------------------+ | |ok=N |Number of acq reqs succeeded | + +-------+-------------------------------------------------------+ -| |nbf=N |Number of acq reqs rejected due to error | -+ +-------+-------------------------------------------------------+ | |oom=N |Number of acq reqs failed on ENOMEM | +--------------+-------+-------------------------------------------------------+ -|Lookups |n=N |Number of lookup calls made on cache backends | +|LRU |n=N |Number of cookies currently on the LRU | + +-------+-------------------------------------------------------+ -| |neg=N |Number of negative lookups made | +| |exp=N |Number of cookies expired off of the LRU | + +-------+-------------------------------------------------------+ -| |pos=N |Number of positive lookups made | +| |rmv=N |Number of cookies removed from the LRU | + +-------+-------------------------------------------------------+ -| |crt=N |Number of objects created by lookup | +| |drp=N |Number of LRU'd cookies relinquished/withdrawn | + +-------+-------------------------------------------------------+ -| |tmo=N |Number of lookups timed out and requeued | +| |at=N |Time till next LRU cull (jiffies) | ++--------------+-------+-------------------------------------------------------+ +|Invals |n=N |Number of invalidations | +--------------+-------+-------------------------------------------------------+ |Updates |n=N |Number of update cookie requests seen | + +-------+-------------------------------------------------------+ -| |nul=N |Number of upd reqs given a NULL parent | +| |rsz=N |Number of resize requests | + +-------+-------------------------------------------------------+ -| |run=N |Number of upd reqs granted CPU time | +| |rsn=N |Number of skipped resize requests | +--------------+-------+-------------------------------------------------------+ |Relinqs |n=N |Number of relinquish cookie requests seen | + +-------+-------------------------------------------------------+ -| |nul=N |Number of rlq reqs given a NULL parent | +| |rtr=N |Number of rlq reqs with retire=true | + +-------+-------------------------------------------------------+ -| |wcr=N |Number of rlq reqs waited on completion of creation | +| |drop=N |Number of cookies no longer blocking re-acquisition | +--------------+-------+-------------------------------------------------------+ -|AttrChg |n=N |Number of attribute changed requests seen | -+ +-------+-------------------------------------------------------+ -| |ok=N |Number of attr changed requests queued | -+ +-------+-------------------------------------------------------+ -| |nbf=N |Number of attr changed rejected -ENOBUFS | +|NoSpace |nwr=N |Number of write requests refused due to lack of space | + +-------+-------------------------------------------------------+ -| |oom=N |Number of attr changed failed -ENOMEM | +| |ncr=N |Number of create requests refused due to lack of space | + +-------+-------------------------------------------------------+ -| |run=N |Number of attr changed ops given CPU time | +| |cull=N |Number of objects culled to make space | +--------------+-------+-------------------------------------------------------+ -|Allocs |n=N |Number of allocation requests seen | +|IO |rd=N |Number of read operations in the cache | + +-------+-------------------------------------------------------+ -| |ok=N |Number of successful alloc reqs | -+ +-------+-------------------------------------------------------+ -| |wt=N |Number of alloc reqs that waited on lookup completion | -+ +-------+-------------------------------------------------------+ -| |nbf=N |Number of alloc reqs rejected -ENOBUFS | -+ +-------+-------------------------------------------------------+ -| |int=N |Number of alloc reqs aborted -ERESTARTSYS | -+ +-------+-------------------------------------------------------+ -| |ops=N |Number of alloc reqs submitted | -+ +-------+-------------------------------------------------------+ -| |owt=N |Number of alloc reqs waited for CPU time | -+ +-------+-------------------------------------------------------+ -| |abt=N |Number of alloc reqs aborted due to object death | -+--------------+-------+-------------------------------------------------------+ -|Retrvls |n=N |Number of retrieval (read) requests seen | -+ +-------+-------------------------------------------------------+ -| |ok=N |Number of successful retr reqs | -+ +-------+-------------------------------------------------------+ -| |wt=N |Number of retr reqs that waited on lookup completion | -+ +-------+-------------------------------------------------------+ -| |nod=N |Number of retr reqs returned -ENODATA | -+ +-------+-------------------------------------------------------+ -| |nbf=N |Number of retr reqs rejected -ENOBUFS | -+ +-------+-------------------------------------------------------+ -| |int=N |Number of retr reqs aborted -ERESTARTSYS | -+ +-------+-------------------------------------------------------+ -| |oom=N |Number of retr reqs failed -ENOMEM | -+ +-------+-------------------------------------------------------+ -| |ops=N |Number of retr reqs submitted | -+ +-------+-------------------------------------------------------+ -| |owt=N |Number of retr reqs waited for CPU time | -+ +-------+-------------------------------------------------------+ -| |abt=N |Number of retr reqs aborted due to object death | -+--------------+-------+-------------------------------------------------------+ -|Stores |n=N |Number of storage (write) requests seen | -+ +-------+-------------------------------------------------------+ -| |ok=N |Number of successful store reqs | -+ +-------+-------------------------------------------------------+ -| |agn=N |Number of store reqs on a page already pending storage | -+ +-------+-------------------------------------------------------+ -| |nbf=N |Number of store reqs rejected -ENOBUFS | -+ +-------+-------------------------------------------------------+ -| |oom=N |Number of store reqs failed -ENOMEM | -+ +-------+-------------------------------------------------------+ -| |ops=N |Number of store reqs submitted | -+ +-------+-------------------------------------------------------+ -| |run=N |Number of store reqs granted CPU time | -+ +-------+-------------------------------------------------------+ -| |pgs=N |Number of pages given store req processing time | -+ +-------+-------------------------------------------------------+ -| |rxd=N |Number of store reqs deleted from tracking tree | -+ +-------+-------------------------------------------------------+ -| |olm=N |Number of store reqs over store limit | -+--------------+-------+-------------------------------------------------------+ -|VmScan |nos=N |Number of release reqs against pages with no | -| | |pending store | -+ +-------+-------------------------------------------------------+ -| |gon=N |Number of release reqs against pages stored by | -| | |time lock granted | -+ +-------+-------------------------------------------------------+ -| |bsy=N |Number of release reqs ignored due to in-progress store| -+ +-------+-------------------------------------------------------+ -| |can=N |Number of page stores cancelled due to release req | -+--------------+-------+-------------------------------------------------------+ -|Ops |pend=N |Number of times async ops added to pending queues | -+ +-------+-------------------------------------------------------+ -| |run=N |Number of times async ops given CPU time | -+ +-------+-------------------------------------------------------+ -| |enq=N |Number of times async ops queued for processing | -+ +-------+-------------------------------------------------------+ -| |can=N |Number of async ops cancelled | -+ +-------+-------------------------------------------------------+ -| |rej=N |Number of async ops rejected due to object | -| | |lookup/create failure | -+ +-------+-------------------------------------------------------+ -| |ini=N |Number of async ops initialised | -+ +-------+-------------------------------------------------------+ -| |dfr=N |Number of async ops queued for deferred release | -+ +-------+-------------------------------------------------------+ -| |rel=N |Number of async ops released | -| | |(should equal ini=N when idle) | -+ +-------+-------------------------------------------------------+ -| |gc=N |Number of deferred-release async ops garbage collected | -+--------------+-------+-------------------------------------------------------+ -|CacheOp |alo=N |Number of in-progress alloc_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |luo=N |Number of in-progress lookup_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |luc=N |Number of in-progress lookup_complete() cache ops | -+ +-------+-------------------------------------------------------+ -| |gro=N |Number of in-progress grab_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |upo=N |Number of in-progress update_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |dro=N |Number of in-progress drop_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |pto=N |Number of in-progress put_object() cache ops | -+ +-------+-------------------------------------------------------+ -| |syn=N |Number of in-progress sync_cache() cache ops | -+ +-------+-------------------------------------------------------+ -| |atc=N |Number of in-progress attr_changed() cache ops | -+ +-------+-------------------------------------------------------+ -| |rap=N |Number of in-progress read_or_alloc_page() cache ops | -+ +-------+-------------------------------------------------------+ -| |ras=N |Number of in-progress read_or_alloc_pages() cache ops | -+ +-------+-------------------------------------------------------+ -| |alp=N |Number of in-progress allocate_page() cache ops | -+ +-------+-------------------------------------------------------+ -| |als=N |Number of in-progress allocate_pages() cache ops | -+ +-------+-------------------------------------------------------+ -| |wrp=N |Number of in-progress write_page() cache ops | -+ +-------+-------------------------------------------------------+ -| |ucp=N |Number of in-progress uncache_page() cache ops | -+ +-------+-------------------------------------------------------+ -| |dsp=N |Number of in-progress dissociate_pages() cache ops | -+--------------+-------+-------------------------------------------------------+ -|CacheEv |nsp=N |Number of object lookups/creations rejected due to | -| | |lack of space | -+ +-------+-------------------------------------------------------+ -| |stl=N |Number of stale objects deleted | -+ +-------+-------------------------------------------------------+ -| |rtr=N |Number of objects retired when relinquished | -+ +-------+-------------------------------------------------------+ -| |cul=N |Number of objects culled | +| |wr=N |Number of write operations in the cache | +--------------+-------+-------------------------------------------------------+ +Netfslib will also add some stats counters of its own. -/proc/fs/fscache/histogram --------------------------- +Cache List +========== - :: +FS-Cache provides a list of cache cookies: - cat /proc/fs/fscache/histogram - JIFS SECS OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS - ===== ===== ========= ========= ========= ========= ========= + /proc/fs/fscache/cookies - This shows the breakdown of the number of times each amount of time - between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The - columns are as follows: +This will look something like:: - ========= ======================================================= - COLUMN TIME MEASUREMENT - ========= ======================================================= - OBJ INST Length of time to instantiate an object - OP RUNS Length of time a call to process an operation took - OBJ RUNS Length of time a call to process an object event took - RETRV DLY Time between an requesting a read and lookup completing - RETRIEVLS Time between beginning and end of a retrieval - ========= ======================================================= + # cat /proc/fs/fscache/caches + CACHE REF VOLS OBJS ACCES S NAME + ======== ===== ===== ===== ===== = =============== + 00000001 2 1 2123 1 A default - Each row shows the number of events that took a particular range of times. - Each step is 1 jiffy in size. The JIFS column indicates the particular - jiffy range covered, and the SECS field the equivalent number of seconds. +where the columns are: + ======= =============================================================== + COLUMN DESCRIPTION + ======= =============================================================== + CACHE Cache cookie debug ID (also appears in traces) + REF Number of references on the cache cookie + VOLS Number of volumes cookies in this cache + OBJS Number of cache objects in use + ACCES Number of accesses pinning the cache + S State + NAME Name of the cache. + ======= =============================================================== + +The state can be (-) Inactive, (P)reparing, (A)ctive, (E)rror or (W)ithdrawing. -Object List +Volume List =========== -If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a -list of all the objects currently allocated and allow them to be viewed -through:: +FS-Cache provides a list of volume cookies: - /proc/fs/fscache/objects + /proc/fs/fscache/volumes This will look something like:: - [root@andromeda ~]# head /proc/fs/fscache/objects - OBJECT PARENT STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA OBJECT_KEY, AUX_DATA - ======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================ - 17e4b 2 ACTV 0 0 0 0 0 0 7b 4 0 0 | NFS.fh DT 0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a - 1693a 2 ACTV 0 0 0 0 0 0 7b 4 0 0 | NFS.fh DT 0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a + VOLUME REF nCOOK ACC FL CACHE KEY + ======== ===== ===== === == =============== ================ + 00000001 55 54 1 00 default afs,example.com,100058 -where the first set of columns before the '|' describe the object: +where the columns are: ======= =============================================================== COLUMN DESCRIPTION ======= =============================================================== - OBJECT Object debugging ID (appears as OBJ%x in some debug messages) - PARENT Debugging ID of parent object - STAT Object state - CHLDN Number of child objects of this object - OPS Number of outstanding operations on this object - OOP Number of outstanding child object management operations - IPR - EX Number of outstanding exclusive operations - READS Number of outstanding read operations - EM Object's event mask - EV Events raised on this object - F Object flags - S Object work item busy state mask (1:pending 2:running) + VOLUME The volume cookie debug ID (also appears in traces) + REF Number of references on the volume cookie + nCOOK Number of cookies in the volume + ACC Number of accesses pinning the cache + FL Flags on the volume cookie + CACHE Name of the cache or "-" + KEY The indexing key for the volume ======= =============================================================== -and the second set of columns describe the object's cookie, if present: - - ================ ====================================================== - COLUMN DESCRIPTION - ================ ====================================================== - NETFS_COOKIE_DEF Name of netfs cookie definition - TY Cookie type (IX - index, DT - data, hex - special) - FL Cookie flags - NETFS_DATA Netfs private data stored in the cookie - OBJECT_KEY Object key } 1 column, with separating comma - AUX_DATA Object aux data } presence may be configured - ================ ====================================================== - -The data shown may be filtered by attaching the a key to an appropriate keyring -before viewing the file. Something like:: - - keyctl add user fscache:objlist <restrictions> @s - -where <restrictions> are a selection of the following letters: - == ========================================================= - K Show hexdump of object key (don't show if not given) - A Show hexdump of object aux data (don't show if not given) - == ========================================================= +Cookie List +=========== -and the following paired letters: +FS-Cache provides a list of cookies: - == ========================================================= - C Show objects that have a cookie - c Show objects that don't have a cookie - B Show objects that are busy - b Show objects that aren't busy - W Show objects that have pending writes - w Show objects that don't have pending writes - R Show objects that have outstanding reads - r Show objects that don't have outstanding reads - S Show objects that have work queued - s Show objects that don't have work queued - == ========================================================= + /proc/fs/fscache/cookies -If neither side of a letter pair is given, then both are implied. For example: +This will look something like:: - keyctl add user fscache:objlist KB @s + # head /proc/fs/fscache/cookies + COOKIE VOLUME REF ACT ACC S FL DEF + ======== ======== === === === = == ================ + 00000435 00000001 1 0 -1 - 08 0000000201d080070000000000000000, 0000000000000000 + 00000436 00000001 1 0 -1 - 00 0000005601d080080000000000000000, 0000000000000051 + 00000437 00000001 1 0 -1 - 08 00023b3001d0823f0000000000000000, 0000000000000000 + 00000438 00000001 1 0 -1 - 08 0000005801d0807b0000000000000000, 0000000000000000 + 00000439 00000001 1 0 -1 - 08 00023b3201d080a10000000000000000, 0000000000000000 + 0000043a 00000001 1 0 -1 - 08 00023b3401d080a30000000000000000, 0000000000000000 + 0000043b 00000001 1 0 -1 - 08 00023b3601d080b30000000000000000, 0000000000000000 + 0000043c 00000001 1 0 -1 - 08 00023b3801d080b40000000000000000, 0000000000000000 -shows objects that are busy, and lists their object keys, but does not dump -their auxiliary data. It also implies "CcWwRrSs", but as 'B' is given, 'b' is -not implied. +where the columns are: -By default all objects and all fields will be shown. + ======= =============================================================== + COLUMN DESCRIPTION + ======= =============================================================== + COOKIE The cookie debug ID (also appears in traces) + VOLUME The parent volume cookie debug ID + REF Number of references on the volume cookie + ACT Number of times the cookie is marked for in use + ACC Number of access pins in the cookie + S State of the cookie + FL Flags on the cookie + DEF Key, auxiliary data + ======= =============================================================== Debugging @@ -549,10 +334,8 @@ This is a bitmask of debugging streams to enable: 3 8 Cookie management Function entry trace 4 16 Function exit trace 5 32 General - 6 64 Page handling Function entry trace - 7 128 Function exit trace - 8 256 General - 9 512 Operation management Function entry trace + 6-8 (Not used) + 9 512 I/O operation management Function entry trace 10 1024 Function exit trace 11 2048 General ======= ======= =============================== ======================= @@ -560,6 +343,6 @@ This is a bitmask of debugging streams to enable: The appropriate set of values should be OR'd together and the result written to the control file. For example:: - echo $((1|8|64)) >/sys/module/fscache/parameters/debug + echo $((1|8|512)) >/sys/module/fscache/parameters/debug will turn on all function entry debugging. diff --git a/Documentation/filesystems/caching/index.rst b/Documentation/filesystems/caching/index.rst index 033da7ac7c6e..df4307124b00 100644 --- a/Documentation/filesystems/caching/index.rst +++ b/Documentation/filesystems/caching/index.rst @@ -7,8 +7,6 @@ Filesystem Caching :maxdepth: 2 fscache - object + netfs-api backend-api cachefiles - netfs-api - operations diff --git a/Documentation/filesystems/caching/netfs-api.rst b/Documentation/filesystems/caching/netfs-api.rst index d9f14b8610ba..f84e9ffdf0b4 100644 --- a/Documentation/filesystems/caching/netfs-api.rst +++ b/Documentation/filesystems/caching/netfs-api.rst @@ -1,896 +1,452 @@ .. SPDX-License-Identifier: GPL-2.0 -=============================== -FS-Cache Network Filesystem API -=============================== +============================== +Network Filesystem Caching API +============================== -There's an API by which a network filesystem can make use of the FS-Cache -facilities. This is based around a number of principles: +Fscache provides an API by which a network filesystem can make use of local +caching facilities. The API is arranged around a number of principles: - (1) Caches can store a number of different object types. There are two main - object types: indices and files. The first is a special type used by - FS-Cache to make finding objects faster and to make retiring of groups of - objects easier. + (1) A cache is logically organised into volumes and data storage objects + within those volumes. - (2) Every index, file or other object is represented by a cookie. This cookie - may or may not have anything associated with it, but the netfs doesn't - need to care. + (2) Volumes and data storage objects are represented by various types of + cookie. - (3) Barring the top-level index (one entry per cached netfs), the index - hierarchy for each netfs is structured according the whim of the netfs. + (3) Cookies have keys that distinguish them from their peers. -This API is declared in <linux/fscache.h>. + (4) Cookies have coherency data that allows a cache to determine if the + cached data is still valid. -.. This document contains the following sections: - - (1) Network filesystem definition - (2) Index definition - (3) Object definition - (4) Network filesystem (un)registration - (5) Cache tag lookup - (6) Index registration - (7) Data file registration - (8) Miscellaneous object registration - (9) Setting the data file size - (10) Page alloc/read/write - (11) Page uncaching - (12) Index and data file consistency - (13) Cookie enablement - (14) Miscellaneous cookie operations - (15) Cookie unregistration - (16) Index invalidation - (17) Data file invalidation - (18) FS-Cache specific page flags. - - -Network Filesystem Definition -============================= - -FS-Cache needs a description of the network filesystem. This is specified -using a record of the following structure:: - - struct fscache_netfs { - uint32_t version; - const char *name; - struct fscache_cookie *primary_index; - ... - }; - -This first two fields should be filled in before registration, and the third -will be filled in by the registration function; any other fields should just be -ignored and are for internal use only. - -The fields are: - - (1) The name of the netfs (used as the key in the toplevel index). - - (2) The version of the netfs (if the name matches but the version doesn't, the - entire in-cache hierarchy for this netfs will be scrapped and begun - afresh). - - (3) The cookie representing the primary index will be allocated according to - another parameter passed into the registration function. - -For example, kAFS (linux/fs/afs/) uses the following definitions to describe -itself:: - - struct fscache_netfs afs_cache_netfs = { - .version = 0, - .name = "afs", - }; - - -Index Definition -================ - -Indices are used for two purposes: - - (1) To aid the finding of a file based on a series of keys (such as AFS's - "cell", "volume ID", "vnode ID"). - - (2) To make it easier to discard a subset of all the files cached based around - a particular key - for instance to mirror the removal of an AFS volume. - -However, since it's unlikely that any two netfs's are going to want to define -their index hierarchies in quite the same way, FS-Cache tries to impose as few -restraints as possible on how an index is structured and where it is placed in -the tree. The netfs can even mix indices and data files at the same level, but -it's not recommended. - -Each index entry consists of a key of indeterminate length plus some auxiliary -data, also of indeterminate length. - -There are some limits on indices: - - (1) Any index containing non-index objects should be restricted to a single - cache. Any such objects created within an index will be created in the - first cache only. The cache in which an index is created can be - controlled by cache tags (see below). - - (2) The entry data must be atomically journallable, so it is limited to about - 400 bytes at present. At least 400 bytes will be available. - - (3) The depth of the index tree should be judged with care as the search - function is recursive. Too many layers will run the kernel out of stack. - - -Object Definition -================= - -To define an object, a structure of the following type should be filled out:: - - struct fscache_cookie_def - { - uint8_t name[16]; - uint8_t type; - - struct fscache_cache_tag *(*select_cache)( - const void *parent_netfs_data, - const void *cookie_netfs_data); - - enum fscache_checkaux (*check_aux)(void *cookie_netfs_data, - const void *data, - uint16_t datalen, - loff_t object_size); - - void (*get_context)(void *cookie_netfs_data, void *context); - - void (*put_context)(void *cookie_netfs_data, void *context); - - void (*mark_pages_cached)(void *cookie_netfs_data, - struct address_space *mapping, - struct pagevec *cached_pvec); - }; - -This has the following fields: - - (1) The type of the object [mandatory]. - - This is one of the following values: - - FSCACHE_COOKIE_TYPE_INDEX - This defines an index, which is a special FS-Cache type. - - FSCACHE_COOKIE_TYPE_DATAFILE - This defines an ordinary data file. - - Any other value between 2 and 255 - This defines an extraordinary object such as an XATTR. - - (2) The name of the object type (NUL terminated unless all 16 chars are used) - [optional]. - - (3) A function to select the cache in which to store an index [optional]. - - This function is invoked when an index needs to be instantiated in a cache - during the instantiation of a non-index object. Only the immediate index - parent for the non-index object will be queried. Any indices above that - in the hierarchy may be stored in multiple caches. This function does not - need to be supplied for any non-index object or any index that will only - have index children. - - If this function is not supplied or if it returns NULL then the first - cache in the parent's list will be chosen, or failing that, the first - cache in the master list. - - (4) A function to check the auxiliary data [optional]. - - This function will be called to check that a match found in the cache for - this object is valid. For instance with AFS it could check the auxiliary - data against the data version number returned by the server to determine - whether the index entry in a cache is still valid. - - If this function is absent, it will be assumed that matching objects in a - cache are always valid. - - The function is also passed the cache's idea of the object size and may - use this to manage coherency also. - - If present, the function should return one of the following values: - - FSCACHE_CHECKAUX_OKAY - - the entry is okay as is - - FSCACHE_CHECKAUX_NEEDS_UPDATE - - the entry requires update - - FSCACHE_CHECKAUX_OBSOLETE - - the entry should be deleted + (5) I/O is done asynchronously where possible. - This function can also be used to extract data from the auxiliary data in - the cache and copy it into the netfs's structures. +This API is used by:: - (5) A pair of functions to manage contexts for the completion callback - [optional]. + #include <linux/fscache.h>. - The cache read/write functions are passed a context which is then passed - to the I/O completion callback function. To ensure this context remains - valid until after the I/O completion is called, two functions may be - provided: one to get an extra reference on the context, and one to drop a - reference to it. - - If the context is not used or is a type of object that won't go out of - scope, then these functions are not required. These functions are not - required for indices as indices may not contain data. These functions may - be called in interrupt context and so may not sleep. - - (6) A function to mark a page as retaining cache metadata [optional]. - - This is called by the cache to indicate that it is retaining in-memory - information for this page and that the netfs should uncache the page when - it has finished. This does not indicate whether there's data on the disk - or not. Note that several pages at once may be presented for marking. - - The PG_fscache bit is set on the pages before this function would be - called, so the function need not be provided if this is sufficient. - - This function is not required for indices as they're not permitted data. - - (7) A function to unmark all the pages retaining cache metadata [mandatory]. - - This is called by FS-Cache to indicate that a backing store is being - unbound from a cookie and that all the marks on the pages should be - cleared to prevent confusion. Note that the cache will have torn down all - its tracking information so that the pages don't need to be explicitly - uncached. - - This function is not required for indices as they're not permitted data. - - -Network Filesystem (Un)registration -=================================== - -The first step is to declare the network filesystem to the cache. This also -involves specifying the layout of the primary index (for AFS, this would be the -"cell" level). - -The registration function is:: - - int fscache_register_netfs(struct fscache_netfs *netfs); - -It just takes a pointer to the netfs definition. It returns 0 or an error as -appropriate. - -For kAFS, registration is done as follows:: - - ret = fscache_register_netfs(&afs_cache_netfs); - -The last step is, of course, unregistration:: - - void fscache_unregister_netfs(struct fscache_netfs *netfs); - - -Cache Tag Lookup -================ - -FS-Cache permits the use of more than one cache. To permit particular index -subtrees to be bound to particular caches, the second step is to look up cache -representation tags. This step is optional; it can be left entirely up to -FS-Cache as to which cache should be used. The problem with doing that is that -FS-Cache will always pick the first cache that was registered. - -To get the representation for a named tag:: - - struct fscache_cache_tag *fscache_lookup_cache_tag(const char *name); - -This takes a text string as the name and returns a representation of a tag. It -will never return an error. It may return a dummy tag, however, if it runs out -of memory; this will inhibit caching with this tag. - -Any representation so obtained must be released by passing it to this function:: - - void fscache_release_cache_tag(struct fscache_cache_tag *tag); +.. This document contains the following sections: -The tag will be retrieved by FS-Cache when it calls the object definition -operation select_cache(). + (1) Overview + (2) Volume registration + (3) Data file registration + (4) Declaring a cookie to be in use + (5) Resizing a data file (truncation) + (6) Data I/O API + (7) Data file coherency + (8) Data file invalidation + (9) Write back resource management + (10) Caching of local modifications + (11) Page release and invalidation + + +Overview +======== + +The fscache hierarchy is organised on two levels from a network filesystem's +point of view. The upper level represents "volumes" and the lower level +represents "data storage objects". These are represented by two types of +cookie, hereafter referred to as "volume cookies" and "cookies". + +A network filesystem acquires a volume cookie for a volume using a volume key, +which represents all the information that defines that volume (e.g. cell name +or server address, volume ID or share name). This must be rendered as a +printable string that can be used as a directory name (ie. no '/' characters +and shouldn't begin with a '.'). The maximum name length is one less than the +maximum size of a filename component (allowing the cache backend one char for +its own purposes). + +A filesystem would typically have a volume cookie for each superblock. + +The filesystem then acquires a cookie for each file within that volume using an +object key. Object keys are binary blobs and only need to be unique within +their parent volume. The cache backend is reponsible for rendering the binary +blob into something it can use and may employ hash tables, trees or whatever to +improve its ability to find an object. This is transparent to the network +filesystem. + +A filesystem would typically have a cookie for each inode, and would acquire it +in iget and relinquish it when evicting the cookie. + +Once it has a cookie, the filesystem needs to mark the cookie as being in use. +This causes fscache to send the cache backend off to look up/create resources +for the cookie in the background, to check its coherency and, if necessary, to +mark the object as being under modification. + +A filesystem would typically "use" the cookie in its file open routine and +unuse it in file release and it needs to use the cookie around calls to +truncate the cookie locally. It *also* needs to use the cookie when the +pagecache becomes dirty and unuse it when writeback is complete. This is +slightly tricky, and provision is made for it. + +When performing a read, write or resize on a cookie, the filesystem must first +begin an operation. This copies the resources into a holding struct and puts +extra pins into the cache to stop cache withdrawal from tearing down the +structures being used. The actual operation can then be issued and conflicting +invalidations can be detected upon completion. + +The filesystem is expected to use netfslib to access the cache, but that's not +actually required and it can use the fscache I/O API directly. + + +Volume Registration +=================== + +The first step for a network filsystem is to acquire a volume cookie for the +volume it wants to access:: + + struct fscache_volume * + fscache_acquire_volume(const char *volume_key, + const char *cache_name, + const void *coherency_data, + size_t coherency_len); + +This function creates a volume cookie with the specified volume key as its name +and notes the coherency data. + +The volume key must be a printable string with no '/' characters in it. It +should begin with the name of the filesystem and should be no longer than 254 +characters. It should uniquely represent the volume and will be matched with +what's stored in the cache. + +The caller may also specify the name of the cache to use. If specified, +fscache will look up or create a cache cookie of that name and will use a cache +of that name if it is online or comes online. If no cache name is specified, +it will use the first cache that comes to hand and set the name to that. + +The specified coherency data is stored in the cookie and will be matched +against coherency data stored on disk. The data pointer may be NULL if no data +is provided. If the coherency data doesn't match, the entire cache volume will +be invalidated. + +This function can return errors such as EBUSY if the volume key is already in +use by an acquired volume or ENOMEM if an allocation failure occured. It may +also return a NULL volume cookie if fscache is not enabled. It is safe to +pass a NULL cookie to any function that takes a volume cookie. This will +cause that function to do nothing. + + +When the network filesystem has finished with a volume, it should relinquish it +by calling:: + + void fscache_relinquish_volume(struct fscache_volume *volume, + const void *coherency_data, + bool invalidate); + +This will cause the volume to be committed or removed, and if sealed the +coherency data will be set to the value supplied. The amount of coherency data +must match the length specified when the volume was acquired. Note that all +data cookies obtained in this volume must be relinquished before the volume is +relinquished. -Index Registration -================== +Data File Registration +====================== -The third step is to inform FS-Cache about part of an index hierarchy that can -be used to locate files. This is done by requesting a cookie for each index in -the path to the file:: +Once it has a volume cookie, a network filesystem can use it to acquire a +cookie for data storage:: struct fscache_cookie * - fscache_acquire_cookie(struct fscache_cookie *parent, - const struct fscache_object_def *def, + fscache_acquire_cookie(struct fscache_volume *volume, + u8 advice, const void *index_key, size_t index_key_len, const void *aux_data, size_t aux_data_len, - void *netfs_data, - loff_t object_size, - bool enable); + loff_t object_size) -This function creates an index entry in the index represented by parent, -filling in the index entry by calling the operations pointed to by def. +This creates the cookie in the volume using the specified index key. The index +key is a binary blob of the given length and must be unique for the volume. +This is saved into the cookie. There are no restrictions on the content, but +its length shouldn't exceed about three quarters of the maximum filename length +to allow for encoding. -A unique key that represents the object within the parent must be pointed to by -index_key and is of length index_key_len. +The caller should also pass in a piece of coherency data in aux_data. A buffer +of size aux_data_len will be allocated and the coherency data copied in. It is +assumed that the size is invariant over time. The coherency data is used to +check the validity of data in the cache. Functions are provided by which the +coherency data can be updated. -An optional blob of auxiliary data that is to be stored within the cache can be -pointed to with aux_data and should be of length aux_data_len. This would -typically be used for storing coherency data. +The file size of the object being cached should also be provided. This may be +used to trim the data and will be stored with the coherency data. -The netfs may pass an arbitrary value in netfs_data and this will be presented -to it in the event of any calling back. This may also be used in tracing or -logging of messages. +This function never returns an error, though it may return a NULL cookie on +allocation failure or if fscache is not enabled. It is safe to pass in a NULL +volume cookie and pass the NULL cookie returned to any function that takes it. +This will cause that function to do nothing. -The cache tracks the size of the data attached to an object and this set to be -object_size. For indices, this should be 0. This value will be passed to the -->check_aux() callback. -Note that this function never returns an error - all errors are handled -internally. It may, however, return NULL to indicate no cookie. It is quite -acceptable to pass this token back to this function as the parent to another -acquisition (or even to the relinquish cookie, read page and write page -functions - see below). +When the network filesystem has finished with a cookie, it should relinquish it +by calling:: -Note also that no indices are actually created in a cache until a non-index -object needs to be created somewhere down the hierarchy. Furthermore, an index -may be created in several different caches independently at different times. -This is all handled transparently, and the netfs doesn't see any of it. + void fscache_relinquish_cookie(struct fscache_cookie *cookie, + bool retire); -A cookie will be created in the disabled state if enabled is false. A cookie -must be enabled to do anything with it. A disabled cookie can be enabled by -calling fscache_enable_cookie() (see below). +This will cause fscache to either commit the storage backing the cookie or +delete it. -For example, with AFS, a cell would be added to the primary index. This index -entry would have a dependent inode containing volume mappings within this cell:: - cell->cache = - fscache_acquire_cookie(afs_cache_netfs.primary_index, - &afs_cell_cache_index_def, - cell->name, strlen(cell->name), - NULL, 0, - cell, 0, true); +Marking A Cookie In-Use +======================= -And then a particular volume could be added to that index by ID, creating -another index for vnodes (AFS inode equivalents):: +Once a cookie has been acquired by a network filesystem, the filesystem should +tell fscache when it intends to use the cookie (typically done on file open) +and should say when it has finished with it (typically on file close):: - volume->cache = - fscache_acquire_cookie(volume->cell->cache, - &afs_volume_cache_index_def, - &volume->vid, sizeof(volume->vid), - NULL, 0, - volume, 0, true); + void fscache_use_cookie(struct fscache_cookie *cookie, + bool will_modify); + void fscache_unuse_cookie(struct fscache_cookie *cookie, + const void *aux_data, + const loff_t *object_size); +The *use* function tells fscache that it will use the cookie and, additionally, +indicate if the user is intending to modify the contents locally. If not yet +done, this will trigger the cache backend to go and gather the resources it +needs to access/store data in the cache. This is done in the background, and +so may not be complete by the time the function returns. -Data File Registration -====================== +The *unuse* function indicates that a filesystem has finished using a cookie. +It optionally updates the stored coherency data and object size and then +decreases the in-use counter. When the last user unuses the cookie, it is +scheduled for garbage collection. If not reused within a short time, the +resources will be released to reduce system resource consumption. -The fourth step is to request a data file be created in the cache. This is -identical to index cookie acquisition. The only difference is that the type in -the object definition should be something other than index type:: +A cookie must be marked in-use before it can be accessed for read, write or +resize - and an in-use mark must be kept whilst there is dirty data in the +pagecache in order to avoid an oops due to trying to open a file during process +exit. - vnode->cache = - fscache_acquire_cookie(volume->cache, - &afs_vnode_cache_object_def, - &key, sizeof(key), - &aux, sizeof(aux), - vnode, vnode->status.size, true); +Note that in-use marks are cumulative. For each time a cookie is marked +in-use, it must be unused. -Miscellaneous Object Registration +Resizing A Data File (Truncation) ================================= -An optional step is to request an object of miscellaneous type be created in -the cache. This is almost identical to index cookie acquisition. The only -difference is that the type in the object definition should be something other -than index type. While the parent object could be an index, it's more likely -it would be some other type of object such as a data file:: - - xattr->cache = - fscache_acquire_cookie(vnode->cache, - &afs_xattr_cache_object_def, - &xattr->name, strlen(xattr->name), - NULL, 0, - xattr, strlen(xattr->val), true); - -Miscellaneous objects might be used to store extended attributes or directory -entries for example. - - -Setting the Data File Size -========================== +If a network filesystem file is resized locally by truncation, the following +should be called to notify the cache:: -The fifth step is to set the physical attributes of the file, such as its size. -This doesn't automatically reserve any space in the cache, but permits the -cache to adjust its metadata for data tracking appropriately:: + void fscache_resize_cookie(struct fscache_cookie *cookie, + loff_t new_size); - int fscache_attr_changed(struct fscache_cookie *cookie); +The caller must have first marked the cookie in-use. The cookie and the new +size are passed in and the cache is synchronously resized. This is expected to +be called from ``->setattr()`` inode operation under the inode lock. -The cache will return -ENOBUFS if there is no backing cache or if there is no -space to allocate any extra metadata required in the cache. -Note that attempts to read or write data pages in the cache over this size may -be rebuffed with -ENOBUFS. +Data I/O API +============ -This operation schedules an attribute adjustment to happen asynchronously at -some point in the future, and as such, it may happen after the function returns -to the caller. The attribute adjustment excludes read and write operations. +To do data I/O operations directly through a cookie, the following functions +are available:: + int fscache_begin_read_operation(struct netfs_cache_resources *cres, + struct fscache_cookie *cookie); + int fscache_read(struct netfs_cache_resources *cres, + loff_t start_pos, + struct iov_iter *iter, + enum netfs_read_from_hole read_hole, + netfs_io_terminated_t term_func, + void *term_func_priv); + int fscache_write(struct netfs_cache_resources *cres, + loff_t start_pos, + struct iov_iter *iter, + netfs_io_terminated_t term_func, + void *term_func_priv); -Page alloc/read/write -===================== +The *begin* function sets up an operation, attaching the resources required to +the cache resources block from the cookie. Assuming it doesn't return an error +(for instance, it will return -ENOBUFS if given a NULL cookie, but otherwise do +nothing), then one of the other two functions can be issued. -And the sixth step is to store and retrieve pages in the cache. There are -three functions that are used to do this. +The *read* and *write* functions initiate a direct-IO operation. Both take the +previously set up cache resources block, an indication of the start file +position, and an I/O iterator that describes buffer and indicates the amount of +data. -Note: +The read function also takes a parameter to indicate how it should handle a +partially populated region (a hole) in the disk content. This may be to ignore +it, skip over an initial hole and place zeros in the buffer or give an error. - (1) A page should not be re-read or re-allocated without uncaching it first. - - (2) A read or allocated page must be uncached when the netfs page is released - from the pagecache. - - (3) A page should only be written to the cache if previous read or allocated. - -This permits the cache to maintain its page tracking in proper order. - - -PAGE READ ---------- - -Firstly, the netfs should ask FS-Cache to examine the caches and read the -contents cached for a particular page of a particular file if present, or else -allocate space to store the contents if not:: +The read and write functions can be given an optional termination function that +will be run on completion:: typedef - void (*fscache_rw_complete_t)(struct page *page, - void *context, - int error); - - int fscache_read_or_alloc_page(struct fscache_cookie *cookie, - struct page *page, - fscache_rw_complete_t end_io_func, - void *context, - gfp_t gfp); - -The cookie argument must specify a cookie for an object that isn't an index, -the page specified will have the data loaded into it (and is also used to -specify the page number), and the gfp argument is used to control how any -memory allocations made are satisfied. - -If the cookie indicates the inode is not cached: - - (1) The function will return -ENOBUFS. - -Else if there's a copy of the page resident in the cache: - - (1) The mark_pages_cached() cookie operation will be called on that page. + void (*netfs_io_terminated_t)(void *priv, ssize_t transferred_or_error, + bool was_async); - (2) The function will submit a request to read the data from the cache's - backing device directly into the page specified. +If a termination function is given, the operation will be run asynchronously +and the termination function will be called upon completion. If not given, the +operation will be run synchronously. Note that in the asynchronous case, it is +possible for the operation to complete before the function returns. - (3) The function will return 0. +Both the read and write functions end the operation when they complete, +detaching any pinned resources. - (4) When the read is complete, end_io_func() will be invoked with: +The read operation will fail with ESTALE if invalidation occurred whilst the +operation was ongoing. - * The netfs data supplied when the cookie was created. - * The page descriptor. +Data File Coherency +=================== - * The context argument passed to the above function. This will be - maintained with the get_context/put_context functions mentioned above. - - * An argument that's 0 on success or negative for an error code. - - If an error occurs, it should be assumed that the page contains no usable - data. fscache_readpages_cancel() may need to be called. - - end_io_func() will be called in process context if the read is results in - an error, but it might be called in interrupt context if the read is - successful. - -Otherwise, if there's not a copy available in cache, but the cache may be able -to store the page: - - (1) The mark_pages_cached() cookie operation will be called on that page. - - (2) A block may be reserved in the cache and attached to the object at the - appropriate place. - - (3) The function will return -ENODATA. - -This function may also return -ENOMEM or -EINTR, in which case it won't have -read any data from the cache. - - -Page Allocate -------------- - -Alternatively, if there's not expected to be any data in the cache for a page -because the file has been extended, a block can simply be allocated instead:: - - int fscache_alloc_page(struct fscache_cookie *cookie, - struct page *page, - gfp_t gfp); - -This is similar to the fscache_read_or_alloc_page() function, except that it -never reads from the cache. It will return 0 if a block has been allocated, -rather than -ENODATA as the other would. One or the other must be performed -before writing to the cache. - -The mark_pages_cached() cookie operation will be called on the page if -successful. - - -Page Write ----------- - -Secondly, if the netfs changes the contents of the page (either due to an -initial download or if a user performs a write), then the page should be -written back to the cache:: - - int fscache_write_page(struct fscache_cookie *cookie, - struct page *page, - loff_t object_size, - gfp_t gfp); - -The cookie argument must specify a data file cookie, the page specified should -contain the data to be written (and is also used to specify the page number), -object_size is the revised size of the object and the gfp argument is used to -control how any memory allocations made are satisfied. - -The page must have first been read or allocated successfully and must not have -been uncached before writing is performed. - -If the cookie indicates the inode is not cached then: - - (1) The function will return -ENOBUFS. - -Else if space can be allocated in the cache to hold this page: - - (1) PG_fscache_write will be set on the page. - - (2) The function will submit a request to write the data to cache's backing - device directly from the page specified. - - (3) The function will return 0. - - (4) When the write is complete PG_fscache_write is cleared on the page and - anyone waiting for that bit will be woken up. - -Else if there's no space available in the cache, -ENOBUFS will be returned. It -is also possible for the PG_fscache_write bit to be cleared when no write took -place if unforeseen circumstances arose (such as a disk error). - -Writing takes place asynchronously. - - -Multiple Page Read ------------------- - -A facility is provided to read several pages at once, as requested by the -readpages() address space operation:: - - int fscache_read_or_alloc_pages(struct fscache_cookie *cookie, - struct address_space *mapping, - struct list_head *pages, - int *nr_pages, - fscache_rw_complete_t end_io_func, - void *context, - gfp_t gfp); - -This works in a similar way to fscache_read_or_alloc_page(), except: - - (1) Any page it can retrieve data for is removed from pages and nr_pages and - dispatched for reading to the disk. Reads of adjacent pages on disk may - be merged for greater efficiency. - - (2) The mark_pages_cached() cookie operation will be called on several pages - at once if they're being read or allocated. - - (3) If there was an general error, then that error will be returned. - - Else if some pages couldn't be allocated or read, then -ENOBUFS will be - returned. - - Else if some pages couldn't be read but were allocated, then -ENODATA will - be returned. - - Otherwise, if all pages had reads dispatched, then 0 will be returned, the - list will be empty and ``*nr_pages`` will be 0. - - (4) end_io_func will be called once for each page being read as the reads - complete. It will be called in process context if error != 0, but it may - be called in interrupt context if there is no error. - -Note that a return of -ENODATA, -ENOBUFS or any other error does not preclude -some of the pages being read and some being allocated. Those pages will have -been marked appropriately and will need uncaching. - - -Cancellation of Unread Pages ----------------------------- - -If one or more pages are passed to fscache_read_or_alloc_pages() but not then -read from the cache and also not read from the underlying filesystem then -those pages will need to have any marks and reservations removed. This can be -done by calling:: - - void fscache_readpages_cancel(struct fscache_cookie *cookie, - struct list_head *pages); - -prior to returning to the caller. The cookie argument should be as passed to -fscache_read_or_alloc_pages(). Every page in the pages list will be examined -and any that have PG_fscache set will be uncached. - - -Page Uncaching -============== - -To uncache a page, this function should be called:: - - void fscache_uncache_page(struct fscache_cookie *cookie, - struct page *page); - -This function permits the cache to release any in-memory representation it -might be holding for this netfs page. This function must be called once for -each page on which the read or write page functions above have been called to -make sure the cache's in-memory tracking information gets torn down. - -Note that pages can't be explicitly deleted from the a data file. The whole -data file must be retired (see the relinquish cookie function below). - -Furthermore, note that this does not cancel the asynchronous read or write -operation started by the read/alloc and write functions, so the page -invalidation functions must use:: - - bool fscache_check_page_write(struct fscache_cookie *cookie, - struct page *page); - -to see if a page is being written to the cache, and:: - - void fscache_wait_on_page_write(struct fscache_cookie *cookie, - struct page *page); - -to wait for it to finish if it is. - - -When releasepage() is being implemented, a special FS-Cache function exists to -manage the heuristics of coping with vmscan trying to eject pages, which may -conflict with the cache trying to write pages to the cache (which may itself -need to allocate memory):: - - bool fscache_maybe_release_page(struct fscache_cookie *cookie, - struct page *page, - gfp_t gfp); - -This takes the netfs cookie, and the page and gfp arguments as supplied to -releasepage(). It will return false if the page cannot be released yet for -some reason and if it returns true, the page has been uncached and can now be -released. - -To make a page available for release, this function may wait for an outstanding -storage request to complete, or it may attempt to cancel the storage request - -in which case the page will not be stored in the cache this time. - - -Bulk Image Page Uncache ------------------------ - -A convenience routine is provided to perform an uncache on all the pages -attached to an inode. This assumes that the pages on the inode correspond on a -1:1 basis with the pages in the cache:: - - void fscache_uncache_all_inode_pages(struct fscache_cookie *cookie, - struct inode *inode); - -This takes the netfs cookie that the pages were cached with and the inode that -the pages are attached to. This function will wait for pages to finish being -written to the cache and for the cache to finish with the page generally. No -error is returned. - - -Index and Data File consistency -=============================== - -To find out whether auxiliary data for an object is up to data within the -cache, the following function can be called:: - - int fscache_check_consistency(struct fscache_cookie *cookie, - const void *aux_data); - -This will call back to the netfs to check whether the auxiliary data associated -with a cookie is correct; if aux_data is non-NULL, it will update the auxiliary -data buffer first. It returns 0 if it is and -ESTALE if it isn't; it may also -return -ENOMEM and -ERESTARTSYS. - -To request an update of the index data for an index or other object, the -following function should be called:: +To request an update of the coherency data and file size on a cookie, the +following should be called:: void fscache_update_cookie(struct fscache_cookie *cookie, - const void *aux_data); - -This function will update the cookie's auxiliary data buffer from aux_data if -that is non-NULL and then schedule this to be stored on disk. The update -method in the parent index definition will be called to transfer the data. - -Note that partial updates may happen automatically at other times, such as when -data blocks are added to a data file object. - - -Cookie Enablement -================= - -Cookies exist in one of two states: enabled and disabled. If a cookie is -disabled, it ignores all attempts to acquire child cookies; check, update or -invalidate its state; allocate, read or write backing pages - though it is -still possible to uncache pages and relinquish the cookie. - -The initial enablement state is set by fscache_acquire_cookie(), but the cookie -can be enabled or disabled later. To disable a cookie, call:: - - void fscache_disable_cookie(struct fscache_cookie *cookie, - const void *aux_data, - bool invalidate); - -If the cookie is not already disabled, this locks the cookie against other -enable and disable ops, marks the cookie as being disabled, discards or -invalidates any backing objects and waits for cessation of activity on any -associated object before unlocking the cookie. - -All possible failures are handled internally. The caller should consider -calling fscache_uncache_all_inode_pages() afterwards to make sure all page -markings are cleared up. - -Cookies can be enabled or reenabled with:: - - void fscache_enable_cookie(struct fscache_cookie *cookie, const void *aux_data, - loff_t object_size, - bool (*can_enable)(void *data), - void *data) - -If the cookie is not already enabled, this locks the cookie against other -enable and disable ops, invokes can_enable() and, if the cookie is not an index -cookie, will begin the procedure of acquiring backing objects. - -The optional can_enable() function is passed the data argument and returns a -ruling as to whether or not enablement should actually be permitted to begin. + const loff_t *object_size); -All possible failures are handled internally. The cookie will only be marked -as enabled if provisional backing objects are allocated. +This will update the cookie's coherency data and/or file size. -The object's data size is updated from object_size and is passed to the -->check_aux() function. -In both cases, the cookie's auxiliary data buffer is updated from aux_data if -that is non-NULL inside the enablement lock before proceeding. - - -Miscellaneous Cookie operations -=============================== +Data File Invalidation +====================== -There are a number of operations that can be used to control cookies: +Sometimes it will be necessary to invalidate an object that contains data. +Typically this will be necessary when the server informs the network filesystem +of a remote third-party change - at which point the filesystem has to throw +away the state and cached data that it had for an file and reload from the +server. - * Cookie pinning:: +To indicate that a cache object should be invalidated, the following should be +called:: - int fscache_pin_cookie(struct fscache_cookie *cookie); - void fscache_unpin_cookie(struct fscache_cookie *cookie); + void fscache_invalidate(struct fscache_cookie *cookie, + const void *aux_data, + loff_t size, + unsigned int flags); - These operations permit data cookies to be pinned into the cache and to - have the pinning removed. They are not permitted on index cookies. +This increases the invalidation counter in the cookie to cause outstanding +reads to fail with -ESTALE, sets the coherency data and file size from the +information supplied, blocks new I/O on the cookie and dispatches the cache to +go and get rid of the old data. - The pinning function will return 0 if successful, -ENOBUFS in the cookie - isn't backed by a cache, -EOPNOTSUPP if the cache doesn't support pinning, - -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or - -EIO if there's any other problem. +Invalidation runs asynchronously in a worker thread so that it doesn't block +too much. - * Data space reservation:: - int fscache_reserve_space(struct fscache_cookie *cookie, loff_t size); +Write-Back Resource Management +============================== - This permits a netfs to request cache space be reserved to store up to the - given amount of a file. It is permitted to ask for more than the current - size of the file to allow for future file expansion. +To write data to the cache from network filesystem writeback, the cache +resources required need to be pinned at the point the modification is made (for +instance when the page is marked dirty) as it's not possible to open a file in +a thread that's exiting. - If size is given as zero then the reservation will be cancelled. +The following facilities are provided to manage this: - The function will return 0 if successful, -ENOBUFS in the cookie isn't - backed by a cache, -EOPNOTSUPP if the cache doesn't support reservations, - -ENOSPC if there isn't enough space to honour the operation, -ENOMEM or - -EIO if there's any other problem. + * An inode flag, ``I_PINNING_FSCACHE_WB``, is provided to indicate that an + in-use is held on the cookie for this inode. It can only be changed if the + the inode lock is held. - Note that this doesn't pin an object in a cache; it can still be culled to - make space if it's not in use. + * A flag, ``unpinned_fscache_wb`` is placed in the ``writeback_control`` + struct that gets set if ``__writeback_single_inode()`` clears + ``I_PINNING_FSCACHE_WB`` because all the dirty pages were cleared. +To support this, the following functions are provided:: -Cookie Unregistration -===================== + int fscache_set_page_dirty(struct page *page, + struct fscache_cookie *cookie); + void fscache_unpin_writeback(struct writeback_control *wbc, + struct fscache_cookie *cookie); + void fscache_clear_inode_writeback(struct fscache_cookie *cookie, + struct inode *inode, + const void *aux); -To get rid of a cookie, this function should be called:: +The *set* function is intended to be called from the filesystem's +``set_page_dirty`` address space operation. If ``I_PINNING_FSCACHE_WB`` is not +set, it sets that flag and increments the use count on the cookie (the caller +must already have called ``fscache_use_cookie()``). - void fscache_relinquish_cookie(struct fscache_cookie *cookie, - const void *aux_data, - bool retire); +The *unpin* function is intended to be called from the filesystem's +``write_inode`` superblock operation. It cleans up after writing by unusing +the cookie if unpinned_fscache_wb is set in the writeback_control struct. -If retire is non-zero, then the object will be marked for recycling, and all -copies of it will be removed from all active caches in which it is present. -Not only that but all child objects will also be retired. +The *clear* function is intended to be called from the netfs's ``evict_inode`` +superblock operation. It must be called *after* +``truncate_inode_pages_final()``, but *before* ``clear_inode()``. This cleans +up any hanging ``I_PINNING_FSCACHE_WB``. It also allows the coherency data to +be updated. -If retire is zero, then the object may be available again when next the -acquisition function is called. Retirement here will overrule the pinning on a -cookie. -The cookie's auxiliary data will be updated from aux_data if that is non-NULL -so that the cache can lazily update it on disk. +Caching of Local Modifications +============================== -One very important note - relinquish must NOT be called for a cookie unless all -the cookies for "child" indices, objects and pages have been relinquished -first. +If a network filesystem has locally modified data that it wants to write to the +cache, it needs to mark the pages to indicate that a write is in progress, and +if the mark is already present, it needs to wait for it to be removed first +(presumably due to an already in-progress operation). This prevents multiple +competing DIO writes to the same storage in the cache. +Firstly, the netfs should determine if caching is available by doing something +like:: -Index Invalidation -================== + bool caching = fscache_cookie_enabled(cookie); -There is no direct way to invalidate an index subtree. To do this, the caller -should relinquish and retire the cookie they have, and then acquire a new one. +If caching is to be attempted, pages should be waited for and then marked using +the following functions provided by the netfs helper library:: + void set_page_fscache(struct page *page); + void wait_on_page_fscache(struct page *page); + int wait_on_page_fscache_killable(struct page *page); -Data File Invalidation -====================== +Once all the pages in the span are marked, the netfs can ask fscache to +schedule a write of that region:: -Sometimes it will be necessary to invalidate an object that contains data. -Typically this will be necessary when the server tells the netfs of a foreign -change - at which point the netfs has to throw away all the state it had for an -inode and reload from the server. + void fscache_write_to_cache(struct fscache_cookie *cookie, + struct address_space *mapping, + loff_t start, size_t len, loff_t i_size, + netfs_io_terminated_t term_func, + void *term_func_priv, + bool caching) -To indicate that a cache object should be invalidated, the following function -can be called:: +And if an error occurs before that point is reached, the marks can be removed +by calling:: - void fscache_invalidate(struct fscache_cookie *cookie); + void fscache_clear_page_bits(struct fscache_cookie *cookie, + struct address_space *mapping, + loff_t start, size_t len, + bool caching) -This can be called with spinlocks held as it defers the work to a thread pool. -All extant storage, retrieval and attribute change ops at this point are -cancelled and discarded. Some future operations will be rejected until the -cache has had a chance to insert a barrier in the operations queue. After -that, operations will be queued again behind the invalidation operation. +In both of these functions, the cookie representing the cache object to be +written to and a pointer to the mapping to which the source pages are attached +are passed in; start and len indicate the size of the region that's going to be +written (it doesn't have to align to page boundaries necessarily, but it does +have to align to DIO boundaries on the backing filesystem). The caching +parameter indicates if caching should be skipped, and if false, the functions +do nothing. -The invalidation operation will perform an attribute change operation and an -auxiliary data update operation as it is very likely these will have changed. +The write function takes some additional parameters: i_size indicates the size +of the netfs file and term_func indicates an optional completion function, to +which term_func_priv will be passed, along with the error or amount written. -Using the following function, the netfs can wait for the invalidation operation -to have reached a point at which it can start submitting ordinary operations -once again:: +Note that the write function will always run asynchronously and will unmark all +the pages upon completion before calling term_func. - void fscache_wait_on_invalidate(struct fscache_cookie *cookie); +Page Release and Invalidation +============================= -FS-cache Specific Page Flag -=========================== +Fscache keeps track of whether we have any data in the cache yet for a cache +object we've just created. It knows it doesn't have to do any reading until it +has done a write and then the page it wrote from has been released by the VM, +after which it *has* to look in the cache. -FS-Cache makes use of a page flag, PG_private_2, for its own purpose. This is -given the alternative name PG_fscache. +To inform fscache that a page might now be in the cache, the following function +should be called from the ``releasepage`` address space op:: -PG_fscache is used to indicate that the page is known by the cache, and that -the cache must be informed if the page is going to go away. It's an indication -to the netfs that the cache has an interest in this page, where an interest may -be a pointer to it, resources allocated or reserved for it, or I/O in progress -upon it. + void fscache_note_page_release(struct fscache_cookie *cookie); -The netfs can use this information in methods such as releasepage() to -determine whether it needs to uncache a page or update it. +if the page has been released (ie. releasepage returned true). -Furthermore, if this bit is set, releasepage() and invalidatepage() operations -will be called on a page to get rid of it, even if PG_private is not set. This -allows caching to attempted on a page before read_cache_pages() to be called -after fscache_read_or_alloc_pages() as the former will try and release pages it -was given under certain circumstances. +Page release and page invalidation should also wait for any mark left on the +page to say that a DIO write is underway from that page:: -This bit does not overlap with such as PG_private. This means that FS-Cache -can be used with a filesystem that uses the block buffering code. + void wait_on_page_fscache(struct page *page); + int wait_on_page_fscache_killable(struct page *page); -There are a number of operations defined on this flag:: - int PageFsCache(struct page *page); - void SetPageFsCache(struct page *page) - void ClearPageFsCache(struct page *page) - int TestSetPageFsCache(struct page *page) - int TestClearPageFsCache(struct page *page) +API Function Reference +====================== -These functions are bit test, bit set, bit clear, bit test and set and bit -test and clear operations on PG_fscache. +.. kernel-doc:: include/linux/fscache.h diff --git a/Documentation/filesystems/caching/object.rst b/Documentation/filesystems/caching/object.rst deleted file mode 100644 index ce0e043ccd33..000000000000 --- a/Documentation/filesystems/caching/object.rst +++ /dev/null @@ -1,313 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -==================================================== -In-Kernel Cache Object Representation and Management -==================================================== - -By: David Howells <dhowells@redhat.com> - -.. Contents: - - (*) Representation - - (*) Object management state machine. - - - Provision of cpu time. - - Locking simplification. - - (*) The set of states. - - (*) The set of events. - - -Representation -============== - -FS-Cache maintains an in-kernel representation of each object that a netfs is -currently interested in. Such objects are represented by the fscache_cookie -struct and are referred to as cookies. - -FS-Cache also maintains a separate in-kernel representation of the objects that -a cache backend is currently actively caching. Such objects are represented by -the fscache_object struct. The cache backends allocate these upon request, and -are expected to embed them in their own representations. These are referred to -as objects. - -There is a 1:N relationship between cookies and objects. A cookie may be -represented by multiple objects - an index may exist in more than one cache - -or even by no objects (it may not be cached). - -Furthermore, both cookies and objects are hierarchical. The two hierarchies -correspond, but the cookies tree is a superset of the union of the object trees -of multiple caches:: - - NETFS INDEX TREE : CACHE 1 : CACHE 2 - : : - : +-----------+ : - +----------->| IObject | : - +-----------+ | : +-----------+ : - | ICookie |-------+ : | : - +-----------+ | : | : +-----------+ - | +------------------------------>| IObject | - | : | : +-----------+ - | : V : | - | : +-----------+ : | - V +----------->| IObject | : | - +-----------+ | : +-----------+ : | - | ICookie |-------+ : | : V - +-----------+ | : | : +-----------+ - | +------------------------------>| IObject | - +-----+-----+ : | : +-----------+ - | | : | : | - V | : V : | - +-----------+ | : +-----------+ : | - | ICookie |------------------------->| IObject | : | - +-----------+ | : +-----------+ : | - | V : | : V - | +-----------+ : | : +-----------+ - | | ICookie |-------------------------------->| IObject | - | +-----------+ : | : +-----------+ - V | : V : | - +-----------+ | : +-----------+ : | - | DCookie |------------------------->| DObject | : | - +-----------+ | : +-----------+ : | - | : : | - +-------+-------+ : : | - | | : : | - V V : : V - +-----------+ +-----------+ : : +-----------+ - | DCookie | | DCookie |------------------------>| DObject | - +-----------+ +-----------+ : : +-----------+ - : : - -In the above illustration, ICookie and IObject represent indices and DCookie -and DObject represent data storage objects. Indices may have representation in -multiple caches, but currently, non-index objects may not. Objects of any type -may also be entirely unrepresented. - -As far as the netfs API goes, the netfs is only actually permitted to see -pointers to the cookies. The cookies themselves and any objects attached to -those cookies are hidden from it. - - -Object Management State Machine -=============================== - -Within FS-Cache, each active object is managed by its own individual state -machine. The state for an object is kept in the fscache_object struct, in -object->state. A cookie may point to a set of objects that are in different -states. - -Each state has an action associated with it that is invoked when the machine -wakes up in that state. There are four logical sets of states: - - (1) Preparation: states that wait for the parent objects to become ready. The - representations are hierarchical, and it is expected that an object must - be created or accessed with respect to its parent object. - - (2) Initialisation: states that perform lookups in the cache and validate - what's found and that create on disk any missing metadata. - - (3) Normal running: states that allow netfs operations on objects to proceed - and that update the state of objects. - - (4) Termination: states that detach objects from their netfs cookies, that - delete objects from disk, that handle disk and system errors and that free - up in-memory resources. - - -In most cases, transitioning between states is in response to signalled events. -When a state has finished processing, it will usually set the mask of events in -which it is interested (object->event_mask) and relinquish the worker thread. -Then when an event is raised (by calling fscache_raise_event()), if the event -is not masked, the object will be queued for processing (by calling -fscache_enqueue_object()). - - -Provision of CPU Time ---------------------- - -The work to be done by the various states was given CPU time by the threads of -the slow work facility. This was used in preference to the workqueue facility -because: - - (1) Threads may be completely occupied for very long periods of time by a - particular work item. These state actions may be doing sequences of - synchronous, journalled disk accesses (lookup, mkdir, create, setxattr, - getxattr, truncate, unlink, rmdir, rename). - - (2) Threads may do little actual work, but may rather spend a lot of time - sleeping on I/O. This means that single-threaded and 1-per-CPU-threaded - workqueues don't necessarily have the right numbers of threads. - - -Locking Simplification ----------------------- - -Because only one worker thread may be operating on any particular object's -state machine at once, this simplifies the locking, particularly with respect -to disconnecting the netfs's representation of a cache object (fscache_cookie) -from the cache backend's representation (fscache_object) - which may be -requested from either end. - - -The Set of States -================= - -The object state machine has a set of states that it can be in. There are -preparation states in which the object sets itself up and waits for its parent -object to transit to a state that allows access to its children: - - (1) State FSCACHE_OBJECT_INIT. - - Initialise the object and wait for the parent object to become active. In - the cache, it is expected that it will not be possible to look an object - up from the parent object, until that parent object itself has been looked - up. - -There are initialisation states in which the object sets itself up and accesses -disk for the object metadata: - - (2) State FSCACHE_OBJECT_LOOKING_UP. - - Look up the object on disk, using the parent as a starting point. - FS-Cache expects the cache backend to probe the cache to see whether this - object is represented there, and if it is, to see if it's valid (coherency - management). - - The cache should call fscache_object_lookup_negative() to indicate lookup - failure for whatever reason, and should call fscache_obtained_object() to - indicate success. - - At the completion of lookup, FS-Cache will let the netfs go ahead with - read operations, no matter whether the file is yet cached. If not yet - cached, read operations will be immediately rejected with ENODATA until - the first known page is uncached - as to that point there can be no data - to be read out of the cache for that file that isn't currently also held - in the pagecache. - - (3) State FSCACHE_OBJECT_CREATING. - - Create an object on disk, using the parent as a starting point. This - happens if the lookup failed to find the object, or if the object's - coherency data indicated what's on disk is out of date. In this state, - FS-Cache expects the cache to create - - The cache should call fscache_obtained_object() if creation completes - successfully, fscache_object_lookup_negative() otherwise. - - At the completion of creation, FS-Cache will start processing write - operations the netfs has queued for an object. If creation failed, the - write ops will be transparently discarded, and nothing recorded in the - cache. - -There are some normal running states in which the object spends its time -servicing netfs requests: - - (4) State FSCACHE_OBJECT_AVAILABLE. - - A transient state in which pending operations are started, child objects - are permitted to advance from FSCACHE_OBJECT_INIT state, and temporary - lookup data is freed. - - (5) State FSCACHE_OBJECT_ACTIVE. - - The normal running state. In this state, requests the netfs makes will be - passed on to the cache. - - (6) State FSCACHE_OBJECT_INVALIDATING. - - The object is undergoing invalidation. When the state comes here, it - discards all pending read, write and attribute change operations as it is - going to clear out the cache entirely and reinitialise it. It will then - continue to the FSCACHE_OBJECT_UPDATING state. - - (7) State FSCACHE_OBJECT_UPDATING. - - The state machine comes here to update the object in the cache from the - netfs's records. This involves updating the auxiliary data that is used - to maintain coherency. - -And there are terminal states in which an object cleans itself up, deallocates -memory and potentially deletes stuff from disk: - - (8) State FSCACHE_OBJECT_LC_DYING. - - The object comes here if it is dying because of a lookup or creation - error. This would be due to a disk error or system error of some sort. - Temporary data is cleaned up, and the parent is released. - - (9) State FSCACHE_OBJECT_DYING. - - The object comes here if it is dying due to an error, because its parent - cookie has been relinquished by the netfs or because the cache is being - withdrawn. - - Any child objects waiting on this one are given CPU time so that they too - can destroy themselves. This object waits for all its children to go away - before advancing to the next state. - -(10) State FSCACHE_OBJECT_ABORT_INIT. - - The object comes to this state if it was waiting on its parent in - FSCACHE_OBJECT_INIT, but its parent died. The object will destroy itself - so that the parent may proceed from the FSCACHE_OBJECT_DYING state. - -(11) State FSCACHE_OBJECT_RELEASING. -(12) State FSCACHE_OBJECT_RECYCLING. - - The object comes to one of these two states when dying once it is rid of - all its children, if it is dying because the netfs relinquished its - cookie. In the first state, the cached data is expected to persist, and - in the second it will be deleted. - -(13) State FSCACHE_OBJECT_WITHDRAWING. - - The object transits to this state if the cache decides it wants to - withdraw the object from service, perhaps to make space, but also due to - error or just because the whole cache is being withdrawn. - -(14) State FSCACHE_OBJECT_DEAD. - - The object transits to this state when the in-memory object record is - ready to be deleted. The object processor shouldn't ever see an object in - this state. - - -The Set of Events ------------------ - -There are a number of events that can be raised to an object state machine: - - FSCACHE_OBJECT_EV_UPDATE - The netfs requested that an object be updated. The state machine will ask - the cache backend to update the object, and the cache backend will ask the - netfs for details of the change through its cookie definition ops. - - FSCACHE_OBJECT_EV_CLEARED - This is signalled in two circumstances: - - (a) when an object's last child object is dropped and - - (b) when the last operation outstanding on an object is completed. - - This is used to proceed from the dying state. - - FSCACHE_OBJECT_EV_ERROR - This is signalled when an I/O error occurs during the processing of some - object. - - FSCACHE_OBJECT_EV_RELEASE, FSCACHE_OBJECT_EV_RETIRE - These are signalled when the netfs relinquishes a cookie it was using. - The event selected depends on whether the netfs asks for the backing - object to be retired (deleted) or retained. - - FSCACHE_OBJECT_EV_WITHDRAW - This is signalled when the cache backend wants to withdraw an object. - This means that the object will have to be detached from the netfs's - cookie. - -Because the withdrawing releasing/retiring events are all handled by the object -state machine, it doesn't matter if there's a collision with both ends trying -to sever the connection at the same time. The state machine can just pick -which one it wants to honour, and that effects the other. diff --git a/Documentation/filesystems/caching/operations.rst b/Documentation/filesystems/caching/operations.rst deleted file mode 100644 index 9983e1675447..000000000000 --- a/Documentation/filesystems/caching/operations.rst +++ /dev/null @@ -1,210 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -================================ -Asynchronous Operations Handling -================================ - -By: David Howells <dhowells@redhat.com> - -.. Contents: - - (*) Overview. - - (*) Operation record initialisation. - - (*) Parameters. - - (*) Procedure. - - (*) Asynchronous callback. - - -Overview -======== - -FS-Cache has an asynchronous operations handling facility that it uses for its -data storage and retrieval routines. Its operations are represented by -fscache_operation structs, though these are usually embedded into some other -structure. - -This facility is available to and expected to be used by the cache backends, -and FS-Cache will create operations and pass them off to the appropriate cache -backend for completion. - -To make use of this facility, <linux/fscache-cache.h> should be #included. - - -Operation Record Initialisation -=============================== - -An operation is recorded in an fscache_operation struct:: - - struct fscache_operation { - union { - struct work_struct fast_work; - struct slow_work slow_work; - }; - unsigned long flags; - fscache_operation_processor_t processor; - ... - }; - -Someone wanting to issue an operation should allocate something with this -struct embedded in it. They should initialise it by calling:: - - void fscache_operation_init(struct fscache_operation *op, - fscache_operation_release_t release); - -with the operation to be initialised and the release function to use. - -The op->flags parameter should be set to indicate the CPU time provision and -the exclusivity (see the Parameters section). - -The op->fast_work, op->slow_work and op->processor flags should be set as -appropriate for the CPU time provision (see the Parameters section). - -FSCACHE_OP_WAITING may be set in op->flags prior to each submission of the -operation and waited for afterwards. - - -Parameters -========== - -There are a number of parameters that can be set in the operation record's flag -parameter. There are three options for the provision of CPU time in these -operations: - - (1) The operation may be done synchronously (FSCACHE_OP_MYTHREAD). A thread - may decide it wants to handle an operation itself without deferring it to - another thread. - - This is, for example, used in read operations for calling readpages() on - the backing filesystem in CacheFiles. Although readpages() does an - asynchronous data fetch, the determination of whether pages exist is done - synchronously - and the netfs does not proceed until this has been - determined. - - If this option is to be used, FSCACHE_OP_WAITING must be set in op->flags - before submitting the operation, and the operating thread must wait for it - to be cleared before proceeding:: - - wait_on_bit(&op->flags, FSCACHE_OP_WAITING, - TASK_UNINTERRUPTIBLE); - - - (2) The operation may be fast asynchronous (FSCACHE_OP_FAST), in which case it - will be given to keventd to process. Such an operation is not permitted - to sleep on I/O. - - This is, for example, used by CacheFiles to copy data from a backing fs - page to a netfs page after the backing fs has read the page in. - - If this option is used, op->fast_work and op->processor must be - initialised before submitting the operation:: - - INIT_WORK(&op->fast_work, do_some_work); - - - (3) The operation may be slow asynchronous (FSCACHE_OP_SLOW), in which case it - will be given to the slow work facility to process. Such an operation is - permitted to sleep on I/O. - - This is, for example, used by FS-Cache to handle background writes of - pages that have just been fetched from a remote server. - - If this option is used, op->slow_work and op->processor must be - initialised before submitting the operation:: - - fscache_operation_init_slow(op, processor) - - -Furthermore, operations may be one of two types: - - (1) Exclusive (FSCACHE_OP_EXCLUSIVE). Operations of this type may not run in - conjunction with any other operation on the object being operated upon. - - An example of this is the attribute change operation, in which the file - being written to may need truncation. - - (2) Shareable. Operations of this type may be running simultaneously. It's - up to the operation implementation to prevent interference between other - operations running at the same time. - - -Procedure -========= - -Operations are used through the following procedure: - - (1) The submitting thread must allocate the operation and initialise it - itself. Normally this would be part of a more specific structure with the - generic op embedded within. - - (2) The submitting thread must then submit the operation for processing using - one of the following two functions:: - - int fscache_submit_op(struct fscache_object *object, - struct fscache_operation *op); - - int fscache_submit_exclusive_op(struct fscache_object *object, - struct fscache_operation *op); - - The first function should be used to submit non-exclusive ops and the - second to submit exclusive ones. The caller must still set the - FSCACHE_OP_EXCLUSIVE flag. - - If successful, both functions will assign the operation to the specified - object and return 0. -ENOBUFS will be returned if the object specified is - permanently unavailable. - - The operation manager will defer operations on an object that is still - undergoing lookup or creation. The operation will also be deferred if an - operation of conflicting exclusivity is in progress on the object. - - If the operation is asynchronous, the manager will retain a reference to - it, so the caller should put their reference to it by passing it to:: - - void fscache_put_operation(struct fscache_operation *op); - - (3) If the submitting thread wants to do the work itself, and has marked the - operation with FSCACHE_OP_MYTHREAD, then it should monitor - FSCACHE_OP_WAITING as described above and check the state of the object if - necessary (the object might have died while the thread was waiting). - - When it has finished doing its processing, it should call - fscache_op_complete() and fscache_put_operation() on it. - - (4) The operation holds an effective lock upon the object, preventing other - exclusive ops conflicting until it is released. The operation can be - enqueued for further immediate asynchronous processing by adjusting the - CPU time provisioning option if necessary, eg:: - - op->flags &= ~FSCACHE_OP_TYPE; - op->flags |= ~FSCACHE_OP_FAST; - - and calling:: - - void fscache_enqueue_operation(struct fscache_operation *op) - - This can be used to allow other things to have use of the worker thread - pools. - - -Asynchronous Callback -===================== - -When used in asynchronous mode, the worker thread pool will invoke the -processor method with a pointer to the operation. This should then get at the -container struct by using container_of():: - - static void fscache_write_op(struct fscache_operation *_op) - { - struct fscache_storage *op = - container_of(_op, struct fscache_storage, op); - ... - } - -The caller holds a reference on the operation, and will invoke -fscache_put_operation() when the processor function returns. The processor -function is at liberty to call fscache_enqueue_operation() or to take extra -references. diff --git a/Documentation/filesystems/netfs_library.rst b/Documentation/filesystems/netfs_library.rst index 375baca7edcd..136f8da3d0e2 100644 --- a/Documentation/filesystems/netfs_library.rst +++ b/Documentation/filesystems/netfs_library.rst @@ -454,7 +454,8 @@ operation table looks like the following:: void *term_func_priv); int (*prepare_write)(struct netfs_cache_resources *cres, - loff_t *_start, size_t *_len, loff_t i_size); + loff_t *_start, size_t *_len, loff_t i_size, + bool no_space_allocated_yet); int (*write)(struct netfs_cache_resources *cres, loff_t start_pos, @@ -515,11 +516,14 @@ The methods defined in the table are: * ``prepare_write()`` - [Required] Called to adjust a write to the cache and check that there is - sufficient space in the cache. The start and length values indicate the - size of the write that netfslib is proposing, and this can be adjusted by - the cache to respect DIO boundaries. The file size is passed for - information. + [Required] Called to prepare a write to the cache to take place. This + involves checking to see whether the cache has sufficient space to honour + the write. ``*_start`` and ``*_len`` indicate the region to be written; the + region can be shrunk or it can be expanded to a page boundary either way as + necessary to align for direct I/O. i_size holds the size of the object and + is provided for reference. no_space_allocated_yet is set to true if the + caller is certain that no data has been written to that region - for example + if it tried to do a read from there already. * ``write()`` |