docs/vm: rename documentation files to .rst

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 257 insertions, 0 deletions

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+.. _page_migration:
+
+==============
+Page migration
+==============
+
+Page migration allows the moving of the physical location of pages between
+nodes in a numa system while the process is running. This means that the
+virtual addresses that the process sees do not change. However, the
+system rearranges the physical location of those pages.
+
+The main intend of page migration is to reduce the latency of memory access
+by moving pages near to the processor where the process accessing that memory
+is running.
+
+Page migration allows a process to manually relocate the node on which its
+pages are located through the MF_MOVE and MF_MOVE_ALL options while setting
+a new memory policy via mbind(). The pages of process can also be relocated
+from another process using the sys_migrate_pages() function call. The
+migrate_pages function call takes two sets of nodes and moves pages of a
+process that are located on the from nodes to the destination nodes.
+Page migration functions are provided by the numactl package by Andi Kleen
+(a version later than 0.9.3 is required. Get it from
+ftp://oss.sgi.com/www/projects/libnuma/download/). numactl provides libnuma
+which provides an interface similar to other numa functionality for page
+migration.  cat ``/proc/<pid>/numa_maps`` allows an easy review of where the
+pages of a process are located. See also the numa_maps documentation in the
+proc(5) man page.
+
+Manual migration is useful if for example the scheduler has relocated
+a process to a processor on a distant node. A batch scheduler or an
+administrator may detect the situation and move the pages of the process
+nearer to the new processor. The kernel itself does only provide
+manual page migration support. Automatic page migration may be implemented
+through user space processes that move pages. A special function call
+"move_pages" allows the moving of individual pages within a process.
+A NUMA profiler may f.e. obtain a log showing frequent off node
+accesses and may use the result to move pages to more advantageous
+locations.
+
+Larger installations usually partition the system using cpusets into
+sections of nodes. Paul Jackson has equipped cpusets with the ability to
+move pages when a task is moved to another cpuset (See
+Documentation/cgroup-v1/cpusets.txt).
+Cpusets allows the automation of process locality. If a task is moved to
+a new cpuset then also all its pages are moved with it so that the
+performance of the process does not sink dramatically. Also the pages
+of processes in a cpuset are moved if the allowed memory nodes of a
+cpuset are changed.
+
+Page migration allows the preservation of the relative location of pages
+within a group of nodes for all migration techniques which will preserve a
+particular memory allocation pattern generated even after migrating a
+process. This is necessary in order to preserve the memory latencies.
+Processes will run with similar performance after migration.
+
+Page migration occurs in several steps. First a high level
+description for those trying to use migrate_pages() from the kernel
+(for userspace usage see the Andi Kleen's numactl package mentioned above)
+and then a low level description of how the low level details work.
+
+In kernel use of migrate_pages()
+================================
+
+1. Remove pages from the LRU.
+
+   Lists of pages to be migrated are generated by scanning over
+   pages and moving them into lists. This is done by
+   calling isolate_lru_page().
+   Calling isolate_lru_page increases the references to the page
+   so that it cannot vanish while the page migration occurs.
+   It also prevents the swapper or other scans to encounter
+   the page.
+
+2. We need to have a function of type new_page_t that can be
+   passed to migrate_pages(). This function should figure out
+   how to allocate the correct new page given the old page.
+
+3. The migrate_pages() function is called which attempts
+   to do the migration. It will call the function to allocate
+   the new page for each page that is considered for
+   moving.
+
+How migrate_pages() works
+=========================
+
+migrate_pages() does several passes over its list of pages. A page is moved
+if all references to a page are removable at the time. The page has
+already been removed from the LRU via isolate_lru_page() and the refcount
+is increased so that the page cannot be freed while page migration occurs.
+
+Steps:
+
+1. Lock the page to be migrated
+
+2. Insure that writeback is complete.
+
+3. Lock the new page that we want to move to. It is locked so that accesses to
+   this (not yet uptodate) page immediately lock while the move is in progress.
+
+4. All the page table references to the page are converted to migration
+   entries. This decreases the mapcount of a page. If the resulting
+   mapcount is not zero then we do not migrate the page. All user space
+   processes that attempt to access the page will now wait on the page lock.
+
+5. The radix tree lock is taken. This will cause all processes trying
+   to access the page via the mapping to block on the radix tree spinlock.
+
+6. The refcount of the page is examined and we back out if references remain
+   otherwise we know that we are the only one referencing this page.
+
+7. The radix tree is checked and if it does not contain the pointer to this
+   page then we back out because someone else modified the radix tree.
+
+8. The new page is prepped with some settings from the old page so that
+   accesses to the new page will discover a page with the correct settings.
+
+9. The radix tree is changed to point to the new page.
+
+10. The reference count of the old page is dropped because the radix tree
+    reference is gone. A reference to the new page is established because
+    the new page is referenced to by the radix tree.
+
+11. The radix tree lock is dropped. With that lookups in the mapping
+    become possible again. Processes will move from spinning on the tree_lock
+    to sleeping on the locked new page.
+
+12. The page contents are copied to the new page.
+
+13. The remaining page flags are copied to the new page.
+
+14. The old page flags are cleared to indicate that the page does
+    not provide any information anymore.
+
+15. Queued up writeback on the new page is triggered.
+
+16. If migration entries were page then replace them with real ptes. Doing
+    so will enable access for user space processes not already waiting for
+    the page lock.
+
+19. The page locks are dropped from the old and new page.
+    Processes waiting on the page lock will redo their page faults
+    and will reach the new page.
+
+20. The new page is moved to the LRU and can be scanned by the swapper
+    etc again.
+
+Non-LRU page migration
+======================
+
+Although original migration aimed for reducing the latency of memory access
+for NUMA, compaction who want to create high-order page is also main customer.
+
+Current problem of the implementation is that it is designed to migrate only
+*LRU* pages. However, there are potential non-lru pages which can be migrated
+in drivers, for example, zsmalloc, virtio-balloon pages.
+
+For virtio-balloon pages, some parts of migration code path have been hooked
+up and added virtio-balloon specific functions to intercept migration logics.
+It's too specific to a driver so other drivers who want to make their pages
+movable would have to add own specific hooks in migration path.
+
+To overclome the problem, VM supports non-LRU page migration which provides
+generic functions for non-LRU movable pages without driver specific hooks
+migration path.
+
+If a driver want to make own pages movable, it should define three functions
+which are function pointers of struct address_space_operations.
+
+1. ``bool (*isolate_page) (struct page *page, isolate_mode_t mode);``
+
+   What VM expects on isolate_page function of driver is to return *true*
+   if driver isolates page successfully. On returing true, VM marks the page
+   as PG_isolated so concurrent isolation in several CPUs skip the page
+   for isolation. If a driver cannot isolate the page, it should return *false*.
+
+   Once page is successfully isolated, VM uses page.lru fields so driver
+   shouldn't expect to preserve values in that fields.
+
+2. ``int (*migratepage) (struct address_space *mapping,``
+|	``struct page *newpage, struct page *oldpage, enum migrate_mode);``
+
+   After isolation, VM calls migratepage of driver with isolated page.
+   The function of migratepage is to move content of the old page to new page
+   and set up fields of struct page newpage. Keep in mind that you should
+   indicate to the VM the oldpage is no longer movable via __ClearPageMovable()
+   under page_lock if you migrated the oldpage successfully and returns
+   MIGRATEPAGE_SUCCESS. If driver cannot migrate the page at the moment, driver
+   can return -EAGAIN. On -EAGAIN, VM will retry page migration in a short time
+   because VM interprets -EAGAIN as "temporal migration failure". On returning
+   any error except -EAGAIN, VM will give up the page migration without retrying
+   in this time.
+
+   Driver shouldn't touch page.lru field VM using in the functions.
+
+3. ``void (*putback_page)(struct page *);``
+
+   If migration fails on isolated page, VM should return the isolated page
+   to the driver so VM calls driver's putback_page with migration failed page.
+   In this function, driver should put the isolated page back to the own data
+   structure.
+
+4. non-lru movable page flags
+
+   There are two page flags for supporting non-lru movable page.
+
+   * PG_movable
+
+     Driver should use the below function to make page movable under page_lock::
+
+	void __SetPageMovable(struct page *page, struct address_space *mapping)
+
+     It needs argument of address_space for registering migration
+     family functions which will be called by VM. Exactly speaking,
+     PG_movable is not a real flag of struct page. Rather than, VM
+     reuses page->mapping's lower bits to represent it.
+
+::
+	#define PAGE_MAPPING_MOVABLE 0x2
+	page->mapping = page->mapping | PAGE_MAPPING_MOVABLE;
+
+     so driver shouldn't access page->mapping directly. Instead, driver should
+     use page_mapping which mask off the low two bits of page->mapping under
+     page lock so it can get right struct address_space.
+
+     For testing of non-lru movable page, VM supports __PageMovable function.
+     However, it doesn't guarantee to identify non-lru movable page because
+     page->mapping field is unified with other variables in struct page.
+     As well, if driver releases the page after isolation by VM, page->mapping
+     doesn't have stable value although it has PAGE_MAPPING_MOVABLE
+     (Look at __ClearPageMovable). But __PageMovable is cheap to catch whether
+     page is LRU or non-lru movable once the page has been isolated. Because
+     LRU pages never can have PAGE_MAPPING_MOVABLE in page->mapping. It is also
+     good for just peeking to test non-lru movable pages before more expensive
+     checking with lock_page in pfn scanning to select victim.
+
+     For guaranteeing non-lru movable page, VM provides PageMovable function.
+     Unlike __PageMovable, PageMovable functions validates page->mapping and
+     mapping->a_ops->isolate_page under lock_page. The lock_page prevents sudden
+     destroying of page->mapping.
+
+     Driver using __SetPageMovable should clear the flag via __ClearMovablePage
+     under page_lock before the releasing the page.
+
+   * PG_isolated
+
+     To prevent concurrent isolation among several CPUs, VM marks isolated page
+     as PG_isolated under lock_page. So if a CPU encounters PG_isolated non-lru
+     movable page, it can skip it. Driver doesn't need to manipulate the flag
+     because VM will set/clear it automatically. Keep in mind that if driver
+     sees PG_isolated page, it means the page have been isolated by VM so it
+     shouldn't touch page.lru field.
+     PG_isolated is alias with PG_reclaim flag so driver shouldn't use the flag
+     for own purpose.
+
+Christoph Lameter, May 8, 2006.
+Minchan Kim, Mar 28, 2016.