linux/Documentation/filesystems/journalling.rst
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   1The Linux Journalling API
   2=========================
   3
   4Overview
   5--------
   6
   7Details
   8~~~~~~~
   9
  10The journalling layer is easy to use. You need to first of all create a
  11journal_t data structure. There are two calls to do this dependent on
  12how you decide to allocate the physical media on which the journal
  13resides. The jbd2_journal_init_inode() call is for journals stored in
  14filesystem inodes, or the jbd2_journal_init_dev() call can be used
  15for journal stored on a raw device (in a continuous range of blocks). A
  16journal_t is a typedef for a struct pointer, so when you are finally
  17finished make sure you call jbd2_journal_destroy() on it to free up
  18any used kernel memory.
  19
  20Once you have got your journal_t object you need to 'mount' or load the
  21journal file. The journalling layer expects the space for the journal
  22was already allocated and initialized properly by the userspace tools.
  23When loading the journal you must call jbd2_journal_load() to process
  24journal contents. If the client file system detects the journal contents
  25does not need to be processed (or even need not have valid contents), it
  26may call jbd2_journal_wipe() to clear the journal contents before
  27calling jbd2_journal_load().
  28
  29Note that jbd2_journal_wipe(..,0) calls
  30jbd2_journal_skip_recovery() for you if it detects any outstanding
  31transactions in the journal and similarly jbd2_journal_load() will
  32call jbd2_journal_recover() if necessary. I would advise reading
  33ext4_load_journal() in fs/ext4/super.c for examples on this stage.
  34
  35Now you can go ahead and start modifying the underlying filesystem.
  36Almost.
  37
  38You still need to actually journal your filesystem changes, this is done
  39by wrapping them into transactions. Additionally you also need to wrap
  40the modification of each of the buffers with calls to the journal layer,
  41so it knows what the modifications you are actually making are. To do
  42this use jbd2_journal_start() which returns a transaction handle.
  43
  44jbd2_journal_start() and its counterpart jbd2_journal_stop(),
  45which indicates the end of a transaction are nestable calls, so you can
  46reenter a transaction if necessary, but remember you must call
  47jbd2_journal_stop() the same number of times as
  48jbd2_journal_start() before the transaction is completed (or more
  49accurately leaves the update phase). Ext4/VFS makes use of this feature to
  50simplify handling of inode dirtying, quota support, etc.
  51
  52Inside each transaction you need to wrap the modifications to the
  53individual buffers (blocks). Before you start to modify a buffer you
  54need to call jbd2_journal_get_create_access() /
  55jbd2_journal_get_write_access() /
  56jbd2_journal_get_undo_access() as appropriate, this allows the
  57journalling layer to copy the unmodified
  58data if it needs to. After all the buffer may be part of a previously
  59uncommitted transaction. At this point you are at last ready to modify a
  60buffer, and once you are have done so you need to call
  61jbd2_journal_dirty_metadata(). Or if you've asked for access to a
  62buffer you now know is now longer required to be pushed back on the
  63device you can call jbd2_journal_forget() in much the same way as you
  64might have used bforget() in the past.
  65
  66A jbd2_journal_flush() may be called at any time to commit and
  67checkpoint all your transactions.
  68
  69Then at umount time , in your put_super() you can then call
  70jbd2_journal_destroy() to clean up your in-core journal object.
  71
  72Unfortunately there a couple of ways the journal layer can cause a
  73deadlock. The first thing to note is that each task can only have a
  74single outstanding transaction at any one time, remember nothing commits
  75until the outermost jbd2_journal_stop(). This means you must complete
  76the transaction at the end of each file/inode/address etc. operation you
  77perform, so that the journalling system isn't re-entered on another
  78journal. Since transactions can't be nested/batched across differing
  79journals, and another filesystem other than yours (say ext4) may be
  80modified in a later syscall.
  81
  82The second case to bear in mind is that jbd2_journal_start() can block
  83if there isn't enough space in the journal for your transaction (based
  84on the passed nblocks param) - when it blocks it merely(!) needs to wait
  85for transactions to complete and be committed from other tasks, so
  86essentially we are waiting for jbd2_journal_stop(). So to avoid
  87deadlocks you must treat jbd2_journal_start() /
  88jbd2_journal_stop() as if they were semaphores and include them in
  89your semaphore ordering rules to prevent
  90deadlocks. Note that jbd2_journal_extend() has similar blocking
  91behaviour to jbd2_journal_start() so you can deadlock here just as
  92easily as on jbd2_journal_start().
  93
  94Try to reserve the right number of blocks the first time. ;-). This will
  95be the maximum number of blocks you are going to touch in this
  96transaction. I advise having a look at at least ext4_jbd.h to see the
  97basis on which ext4 uses to make these decisions.
  98
  99Another wriggle to watch out for is your on-disk block allocation
 100strategy. Why? Because, if you do a delete, you need to ensure you
 101haven't reused any of the freed blocks until the transaction freeing
 102these blocks commits. If you reused these blocks and crash happens,
 103there is no way to restore the contents of the reallocated blocks at the
 104end of the last fully committed transaction. One simple way of doing
 105this is to mark blocks as free in internal in-memory block allocation
 106structures only after the transaction freeing them commits. Ext4 uses
 107journal commit callback for this purpose.
 108
 109With journal commit callbacks you can ask the journalling layer to call
 110a callback function when the transaction is finally committed to disk,
 111so that you can do some of your own management. You ask the journalling
 112layer for calling the callback by simply setting
 113``journal->j_commit_callback`` function pointer and that function is
 114called after each transaction commit. You can also use
 115``transaction->t_private_list`` for attaching entries to a transaction
 116that need processing when the transaction commits.
 117
 118JBD2 also provides a way to block all transaction updates via
 119jbd2_journal_lock_updates() /
 120jbd2_journal_unlock_updates(). Ext4 uses this when it wants a
 121window with a clean and stable fs for a moment. E.g.
 122
 123::
 124
 125
 126        jbd2_journal_lock_updates() //stop new stuff happening..
 127        jbd2_journal_flush()        // checkpoint everything.
 128        ..do stuff on stable fs
 129        jbd2_journal_unlock_updates() // carry on with filesystem use.
 130
 131The opportunities for abuse and DOS attacks with this should be obvious,
 132if you allow unprivileged userspace to trigger codepaths containing
 133these calls.
 134
 135Fast commits
 136~~~~~~~~~~~~
 137
 138JBD2 to also allows you to perform file-system specific delta commits known as
 139fast commits. In order to use fast commits, you will need to set following
 140callbacks that perform correspodning work:
 141
 142`journal->j_fc_cleanup_cb`: Cleanup function called after every full commit and
 143fast commit.
 144
 145`journal->j_fc_replay_cb`: Replay function called for replay of fast commit
 146blocks.
 147
 148File system is free to perform fast commits as and when it wants as long as it
 149gets permission from JBD2 to do so by calling the function
 150:c:func:`jbd2_fc_begin_commit()`. Once a fast commit is done, the client
 151file  system should tell JBD2 about it by calling
 152:c:func:`jbd2_fc_end_commit()`. If file system wants JBD2 to perform a full
 153commit immediately after stopping the fast commit it can do so by calling
 154:c:func:`jbd2_fc_end_commit_fallback()`. This is useful if fast commit operation
 155fails for some reason and the only way to guarantee consistency is for JBD2 to
 156perform the full traditional commit.
 157
 158JBD2 helper functions to manage fast commit buffers. File system can use
 159:c:func:`jbd2_fc_get_buf()` and :c:func:`jbd2_fc_wait_bufs()` to allocate
 160and wait on IO completion of fast commit buffers.
 161
 162Currently, only Ext4 implements fast commits. For details of its implementation
 163of fast commits, please refer to the top level comments in
 164fs/ext4/fast_commit.c.
 165
 166Summary
 167~~~~~~~
 168
 169Using the journal is a matter of wrapping the different context changes,
 170being each mount, each modification (transaction) and each changed
 171buffer to tell the journalling layer about them.
 172
 173Data Types
 174----------
 175
 176The journalling layer uses typedefs to 'hide' the concrete definitions
 177of the structures used. As a client of the JBD2 layer you can just rely
 178on the using the pointer as a magic cookie of some sort. Obviously the
 179hiding is not enforced as this is 'C'.
 180
 181Structures
 182~~~~~~~~~~
 183
 184.. kernel-doc:: include/linux/jbd2.h
 185   :internal:
 186
 187Functions
 188---------
 189
 190The functions here are split into two groups those that affect a journal
 191as a whole, and those which are used to manage transactions
 192
 193Journal Level
 194~~~~~~~~~~~~~
 195
 196.. kernel-doc:: fs/jbd2/journal.c
 197   :export:
 198
 199.. kernel-doc:: fs/jbd2/recovery.c
 200   :internal:
 201
 202Transasction Level
 203~~~~~~~~~~~~~~~~~~
 204
 205.. kernel-doc:: fs/jbd2/transaction.c
 206
 207See also
 208--------
 209
 210`Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen
 211Tweedie <http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz>`__
 212
 213`Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen
 214Tweedie <http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html>`__
 215
 216