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 jbd2_journal_init_inode() call is for journals stored in
  filesystem inodes, or the 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 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 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 jbd2_journal_wipe() to clear the journal contents before
  calling jbd2_journal_load().

  
  Note that jbd2_journal_wipe(..,0) calls

  jbd2_journal_skip_recovery() for you if it detects any outstanding
  transactions in the journal and similarly jbd2_journal_load() will
  call jbd2_journal_recover() if necessary. I would advise reading
  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 jbd2_journal_start() which returns a transaction handle.

  jbd2_journal_start() and its counterpart 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

  jbd2_journal_stop() the same number of times as
  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 jbd2_journal_get_create_access() /
  jbd2_journal_get_write_access() /
  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

  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 jbd2_journal_forget() in much the same way as you
  might have used bforget() in the past.

  A jbd2_journal_flush() may be called at any time to commit and

  checkpoint all your transactions.

  Then at umount time , in your put_super() you can then call
  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 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 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 jbd2_journal_stop(). So to avoid
  deadlocks you must treat jbd2_journal_start() /
  jbd2_journal_stop() as if they were semaphores and include them in

  your semaphore ordering rules to prevent

  deadlocks. Note that jbd2_journal_extend() has similar blocking
  behaviour to jbd2_journal_start() so you can deadlock here just as
  easily as on 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

  jbd2_journal_lock_updates() /
  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.

  Fast commits
  ~~~~~~~~~~~~
  
  JBD2 to also allows you to perform file-system specific delta commits known as

  fast commits. In order to use fast commits, you will need to set following
  callbacks that perform correspodning work:

  
  `journal->j_fc_cleanup_cb`: Cleanup function called after every full commit and
  fast commit.
  
  `journal->j_fc_replay_cb`: Replay function called for replay of fast commit
  blocks.
  
  File system is free to perform fast commits as and when it wants as long as it
  gets permission from JBD2 to do so by calling the function
  :c:func:`jbd2_fc_begin_commit()`. Once a fast commit is done, the client
  file  system should tell JBD2 about it by calling
  :c:func:`jbd2_fc_end_commit()`. If file system wants JBD2 to perform a full
  commit immediately after stopping the fast commit it can do so by calling
  :c:func:`jbd2_fc_end_commit_fallback()`. This is useful if fast commit operation
  fails for some reason and the only way to guarantee consistency is for JBD2 to
  perform the full traditional commit.
  
  JBD2 helper functions to manage fast commit buffers. File system can use
  :c:func:`jbd2_fc_get_buf()` and :c:func:`jbd2_fc_wait_bufs()` to allocate
  and wait on IO completion of fast commit buffers.
  
  Currently, only Ext4 implements fast commits. For details of its implementation
  of fast commits, please refer to the top level comments in
  fs/ext4/fast_commit.c.

  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>`__