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, 184 insertions, 0 deletions

diff --git a/Documentation/filesystems/journalling.rst b/Documentation/filesystems/journalling.rst
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+The Linux Journalling API
+=========================
+
+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>`__
+