linux-next/Documentation/filesystems/journalling.rst

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>`__