Documentation/filesystems/journalling.rst - linux - Git at Google

 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 corresponding 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 the 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:

 Transaction 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>`__
	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 corresponding 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 the 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:

	Transaction 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>`__