docs: Bring some order to filesystem documentation

Documentation/filesystems is, like much of the rest of the kernel's
documentation, a jumble of unorganized information.  Split the
documentation into categories and try to bring some order to the top-level
index.rst files.  No text changes other than a few section-introductory
blurbs; this is all just moving stuff around.

Cc: linux-fsdevel@vger.kernel.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 24 insertions, 370 deletions

diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 61d2441b25d5..1131c34d77f6 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -1,389 +1,43 @@
-=====================
-Linux Filesystems API
-=====================
+===============================
+Filesystems in the Linux kernel
+===============================
 
-The Linux VFS
-=============
+This under-development manual will, some glorious day, provide
+comprehensive information on how the Linux virtual filesystem (VFS) layer
+works, along with the filesystems that sit below it.  For now, what we have
+can be found below.
 
-The Filesystem types
---------------------
-
-.. kernel-doc:: include/linux/fs.h
-   :internal:
-
-The Directory Cache
--------------------
-
-.. kernel-doc:: fs/dcache.c
-   :export:
-
-.. kernel-doc:: include/linux/dcache.h
-   :internal:
-
-Inode Handling
---------------
-
-.. kernel-doc:: fs/inode.c
-   :export:
-
-.. kernel-doc:: fs/bad_inode.c
-   :export:
-
-Registration and Superblocks
-----------------------------
-
-.. kernel-doc:: fs/super.c
-   :export:
-
-File Locks
-----------
-
-.. kernel-doc:: fs/locks.c
-   :export:
-
-.. kernel-doc:: fs/locks.c
-   :internal:
-
-Other Functions
----------------
-
-.. kernel-doc:: fs/mpage.c
-   :export:
-
-.. kernel-doc:: fs/namei.c
-   :export:
-
-.. kernel-doc:: fs/buffer.c
-   :export:
-
-.. kernel-doc:: block/bio.c
-   :export:
-
-.. kernel-doc:: fs/seq_file.c
-   :export:
-
-.. kernel-doc:: fs/filesystems.c
-   :export:
-
-.. kernel-doc:: fs/fs-writeback.c
-   :export:
-
-.. kernel-doc:: fs/block_dev.c
-   :export:
-
-.. kernel-doc:: fs/anon_inodes.c
-   :export:
-
-.. kernel-doc:: fs/attr.c
-   :export:
-
-.. kernel-doc:: fs/d_path.c
-   :export:
-
-.. kernel-doc:: fs/dax.c
-   :export:
-
-.. kernel-doc:: fs/direct-io.c
-   :export:
-
-.. kernel-doc:: fs/file_table.c
-   :export:
-
-.. kernel-doc:: fs/libfs.c
-   :export:
-
-.. kernel-doc:: fs/posix_acl.c
-   :export:
-
-.. kernel-doc:: fs/stat.c
-   :export:
-
-.. kernel-doc:: fs/sync.c
-   :export:
-
-.. kernel-doc:: fs/xattr.c
-   :export:
-
-The proc filesystem
-===================
-
-sysctl interface
-----------------
-
-.. kernel-doc:: kernel/sysctl.c
-   :export:
-
-proc filesystem interface
--------------------------
-
-.. kernel-doc:: fs/proc/base.c
-   :internal:
-
-Events based on file descriptors
-================================
-
-.. kernel-doc:: fs/eventfd.c
-   :export:
-
-The Filesystem for Exporting Kernel Objects
-===========================================
-
-.. kernel-doc:: fs/sysfs/file.c
-   :export:
-
-.. kernel-doc:: fs/sysfs/symlink.c
-   :export:
-
-The debugfs filesystem
+Core VFS documentation
 ======================
 
-debugfs interface
------------------
+See these manuals for documentation about the VFS layer itself and how its
+algorithms work.
 
-.. kernel-doc:: fs/debugfs/inode.c
-   :export:
+.. toctree::
+   :maxdepth: 2
 
-.. kernel-doc:: fs/debugfs/file.c
-   :export:
+   path-lookup.rst
+   api-summary
+   splice
 
-The Linux Journalling API
+Filesystem support layers
 =========================
 
-Overview
---------
-
-Details
-~~~~~~~
-
-The journalling layer is easy to use. You need to first of all create a
-journal_t data structure. There are two calls to do this dependent on
-how you decide to allocate the physical media on which the journal
-resides. The :c:func:`jbd2_journal_init_inode` call is for journals stored in
-filesystem inodes, or the :c:func:`jbd2_journal_init_dev` call can be used
-for journal stored on a raw device (in a continuous range of blocks). A
-journal_t is a typedef for a struct pointer, so when you are finally
-finished make sure you call :c:func:`jbd2_journal_destroy` on it to free up
-any used kernel memory.
-
-Once you have got your journal_t object you need to 'mount' or load the
-journal file. The journalling layer expects the space for the journal
-was already allocated and initialized properly by the userspace tools.
-When loading the journal you must call :c:func:`jbd2_journal_load` to process
-journal contents. If the client file system detects the journal contents
-does not need to be processed (or even need not have valid contents), it
-may call :c:func:`jbd2_journal_wipe` to clear the journal contents before
-calling :c:func:`jbd2_journal_load`.
-
-Note that jbd2_journal_wipe(..,0) calls
-:c:func:`jbd2_journal_skip_recovery` for you if it detects any outstanding
-transactions in the journal and similarly :c:func:`jbd2_journal_load` will
-call :c:func:`jbd2_journal_recover` if necessary. I would advise reading
-:c:func:`ext4_load_journal` in fs/ext4/super.c for examples on this stage.
-
-Now you can go ahead and start modifying the underlying filesystem.
-Almost.
-
-You still need to actually journal your filesystem changes, this is done
-by wrapping them into transactions. Additionally you also need to wrap
-the modification of each of the buffers with calls to the journal layer,
-so it knows what the modifications you are actually making are. To do
-this use :c:func:`jbd2_journal_start` which returns a transaction handle.
-
-:c:func:`jbd2_journal_start` and its counterpart :c:func:`jbd2_journal_stop`,
-which indicates the end of a transaction are nestable calls, so you can
-reenter a transaction if necessary, but remember you must call
-:c:func:`jbd2_journal_stop` the same number of times as
-:c:func:`jbd2_journal_start` before the transaction is completed (or more
-accurately leaves the update phase). Ext4/VFS makes use of this feature to
-simplify handling of inode dirtying, quota support, etc.
-
-Inside each transaction you need to wrap the modifications to the
-individual buffers (blocks). Before you start to modify a buffer you
-need to call :c:func:`jbd2_journal_get_create_access()` /
-:c:func:`jbd2_journal_get_write_access()` /
-:c:func:`jbd2_journal_get_undo_access()` as appropriate, this allows the
-journalling layer to copy the unmodified
-data if it needs to. After all the buffer may be part of a previously
-uncommitted transaction. At this point you are at last ready to modify a
-buffer, and once you are have done so you need to call
-:c:func:`jbd2_journal_dirty_metadata`. Or if you've asked for access to a
-buffer you now know is now longer required to be pushed back on the
-device you can call :c:func:`jbd2_journal_forget` in much the same way as you
-might have used :c:func:`bforget` in the past.
-
-A :c:func:`jbd2_journal_flush` may be called at any time to commit and
-checkpoint all your transactions.
-
-Then at umount time , in your :c:func:`put_super` you can then call
-:c:func:`jbd2_journal_destroy` to clean up your in-core journal object.
-
-Unfortunately there a couple of ways the journal layer can cause a
-deadlock. The first thing to note is that each task can only have a
-single outstanding transaction at any one time, remember nothing commits
-until the outermost :c:func:`jbd2_journal_stop`. This means you must complete
-the transaction at the end of each file/inode/address etc. operation you
-perform, so that the journalling system isn't re-entered on another
-journal. Since transactions can't be nested/batched across differing
-journals, and another filesystem other than yours (say ext4) may be
-modified in a later syscall.
-
-The second case to bear in mind is that :c:func:`jbd2_journal_start` can block
-if there isn't enough space in the journal for your transaction (based
-on the passed nblocks param) - when it blocks it merely(!) needs to wait
-for transactions to complete and be committed from other tasks, so
-essentially we are waiting for :c:func:`jbd2_journal_stop`. So to avoid
-deadlocks you must treat :c:func:`jbd2_journal_start` /
-:c:func:`jbd2_journal_stop` as if they were semaphores and include them in
-your semaphore ordering rules to prevent
-deadlocks. Note that :c:func:`jbd2_journal_extend` has similar blocking
-behaviour to :c:func:`jbd2_journal_start` so you can deadlock here just as
-easily as on :c:func:`jbd2_journal_start`.
-
-Try to reserve the right number of blocks the first time. ;-). This will
-be the maximum number of blocks you are going to touch in this
-transaction. I advise having a look at at least ext4_jbd.h to see the
-basis on which ext4 uses to make these decisions.
-
-Another wriggle to watch out for is your on-disk block allocation
-strategy. Why? Because, if you do a delete, you need to ensure you
-haven't reused any of the freed blocks until the transaction freeing
-these blocks commits. If you reused these blocks and crash happens,
-there is no way to restore the contents of the reallocated blocks at the
-end of the last fully committed transaction. One simple way of doing
-this is to mark blocks as free in internal in-memory block allocation
-structures only after the transaction freeing them commits. Ext4 uses
-journal commit callback for this purpose.
-
-With journal commit callbacks you can ask the journalling layer to call
-a callback function when the transaction is finally committed to disk,
-so that you can do some of your own management. You ask the journalling
-layer for calling the callback by simply setting
-``journal->j_commit_callback`` function pointer and that function is
-called after each transaction commit. You can also use
-``transaction->t_private_list`` for attaching entries to a transaction
-that need processing when the transaction commits.
-
-JBD2 also provides a way to block all transaction updates via
-:c:func:`jbd2_journal_lock_updates()` /
-:c:func:`jbd2_journal_unlock_updates()`. Ext4 uses this when it wants a
-window with a clean and stable fs for a moment. E.g.
-
-::
-
-
-        jbd2_journal_lock_updates() //stop new stuff happening..
-        jbd2_journal_flush()        // checkpoint everything.
-        ..do stuff on stable fs
-        jbd2_journal_unlock_updates() // carry on with filesystem use.
-
-The opportunities for abuse and DOS attacks with this should be obvious,
-if you allow unprivileged userspace to trigger codepaths containing
-these calls.
-
-Summary
-~~~~~~~
-
-Using the journal is a matter of wrapping the different context changes,
-being each mount, each modification (transaction) and each changed
-buffer to tell the journalling layer about them.
-
-Data Types
-----------
-
-The journalling layer uses typedefs to 'hide' the concrete definitions
-of the structures used. As a client of the JBD2 layer you can just rely
-on the using the pointer as a magic cookie of some sort. Obviously the
-hiding is not enforced as this is 'C'.
-
-Structures
-~~~~~~~~~~
-
-.. kernel-doc:: include/linux/jbd2.h
-   :internal:
-
-Functions
----------
-
-The functions here are split into two groups those that affect a journal
-as a whole, and those which are used to manage transactions
-
-Journal Level
-~~~~~~~~~~~~~
-
-.. kernel-doc:: fs/jbd2/journal.c
-   :export:
-
-.. kernel-doc:: fs/jbd2/recovery.c
-   :internal:
-
-Transasction Level
-~~~~~~~~~~~~~~~~~~
-
-.. kernel-doc:: fs/jbd2/transaction.c
-
-See also
---------
-
-`Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen
-Tweedie <http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz>`__
-
-`Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen
-Tweedie <http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html>`__
-
-splice API
-==========
-
-splice is a method for moving blocks of data around inside the kernel,
-without continually transferring them between the kernel and user space.
-
-.. kernel-doc:: fs/splice.c
-
-pipes API
-=========
-
-Pipe interfaces are all for in-kernel (builtin image) use. They are not
-exported for use by modules.
-
-.. kernel-doc:: include/linux/pipe_fs_i.h
-   :internal:
-
-.. kernel-doc:: fs/pipe.c
-
-Encryption API
-==============
-
-A library which filesystems can hook into to support transparent
-encryption of files and directories.
+Documentation for the support code within the filesystem layer for use in
+filesystem implementations.
 
 .. toctree::
-    :maxdepth: 2
-
-    fscrypt
-
-Pathname lookup
-===============
-
-
-This write-up is based on three articles published at lwn.net:
+   :maxdepth: 2
 
-- <https://lwn.net/Articles/649115/> Pathname lookup in Linux
-- <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
-- <https://lwn.net/Articles/650786/> A walk among the symlinks
+   journalling
+   fscrypt
 
-Written by Neil Brown with help from Al Viro and Jon Corbet.
-It has subsequently been updated to reflect changes in the kernel
-including:
+Filesystem-specific documentation
+=================================
 
-- per-directory parallel name lookup.
+Documentation for individual filesystem types can be found here.
 
 .. toctree::
    :maxdepth: 2
 
-   path-lookup.rst
-
-binderfs
-========
-
-.. toctree::
-
    binderfs.rst