mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-26 13:44:15 +08:00
37e0a30e94
Now that jbd2_fc_init is dropped, drop its docs too. Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com> Link: https://lore.kernel.org/r/20201106035911.1942128-8-harshadshirwadkar@gmail.com Signed-off-by: Theodore Ts'o <tytso@mit.edu>
216 lines
9.0 KiB
ReStructuredText
216 lines
9.0 KiB
ReStructuredText
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>`__
|
|
|