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diff --git a/Documentation/filesystems/journalling.rst b/Documentation/filesystems/journalling.rst new file mode 100644 index 000000000000..58ce6b395206 --- /dev/null +++ b/Documentation/filesystems/journalling.rst @@ -0,0 +1,184 @@ +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>`__ + |