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>`__