Filesystems: Documentation: Replace deprecated :c:func: Usage
Replace :c:func: with func() as the previous usage is deprecated. Signed-off-by: Puranjay Mohan <puranjay12@gmail.com> Link: https://lore.kernel.org/r/20200810184828.29297-1-puranjay12@gmail.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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@ -10,27 +10,27 @@ Details
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The journalling layer is easy to use. You need to first of all create a
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journal_t data structure. There are two calls to do this dependent on
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how you decide to allocate the physical media on which the journal
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resides. The :c:func:`jbd2_journal_init_inode` call is for journals stored in
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filesystem inodes, or the :c:func:`jbd2_journal_init_dev` call can be used
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resides. The jbd2_journal_init_inode() call is for journals stored in
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filesystem inodes, or the jbd2_journal_init_dev() call can be used
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for journal stored on a raw device (in a continuous range of blocks). A
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journal_t is a typedef for a struct pointer, so when you are finally
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finished make sure you call :c:func:`jbd2_journal_destroy` on it to free up
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finished make sure you call jbd2_journal_destroy() on it to free up
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any used kernel memory.
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Once you have got your journal_t object you need to 'mount' or load the
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journal file. The journalling layer expects the space for the journal
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was already allocated and initialized properly by the userspace tools.
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When loading the journal you must call :c:func:`jbd2_journal_load` to process
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When loading the journal you must call jbd2_journal_load() to process
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journal contents. If the client file system detects the journal contents
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does not need to be processed (or even need not have valid contents), it
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may call :c:func:`jbd2_journal_wipe` to clear the journal contents before
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calling :c:func:`jbd2_journal_load`.
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may call jbd2_journal_wipe() to clear the journal contents before
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calling jbd2_journal_load().
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Note that jbd2_journal_wipe(..,0) calls
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:c:func:`jbd2_journal_skip_recovery` for you if it detects any outstanding
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transactions in the journal and similarly :c:func:`jbd2_journal_load` will
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call :c:func:`jbd2_journal_recover` if necessary. I would advise reading
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:c:func:`ext4_load_journal` in fs/ext4/super.c for examples on this stage.
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jbd2_journal_skip_recovery() for you if it detects any outstanding
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transactions in the journal and similarly jbd2_journal_load() will
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call jbd2_journal_recover() if necessary. I would advise reading
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ext4_load_journal() in fs/ext4/super.c for examples on this stage.
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Now you can go ahead and start modifying the underlying filesystem.
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Almost.
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@ -39,57 +39,57 @@ You still need to actually journal your filesystem changes, this is done
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by wrapping them into transactions. Additionally you also need to wrap
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the modification of each of the buffers with calls to the journal layer,
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so it knows what the modifications you are actually making are. To do
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this use :c:func:`jbd2_journal_start` which returns a transaction handle.
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this use jbd2_journal_start() which returns a transaction handle.
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:c:func:`jbd2_journal_start` and its counterpart :c:func:`jbd2_journal_stop`,
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jbd2_journal_start() and its counterpart jbd2_journal_stop(),
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which indicates the end of a transaction are nestable calls, so you can
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reenter a transaction if necessary, but remember you must call
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:c:func:`jbd2_journal_stop` the same number of times as
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:c:func:`jbd2_journal_start` before the transaction is completed (or more
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jbd2_journal_stop() the same number of times as
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jbd2_journal_start() before the transaction is completed (or more
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accurately leaves the update phase). Ext4/VFS makes use of this feature to
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simplify handling of inode dirtying, quota support, etc.
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Inside each transaction you need to wrap the modifications to the
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individual buffers (blocks). Before you start to modify a buffer you
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need to call :c:func:`jbd2_journal_get_create_access()` /
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:c:func:`jbd2_journal_get_write_access()` /
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:c:func:`jbd2_journal_get_undo_access()` as appropriate, this allows the
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need to call jbd2_journal_get_create_access() /
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jbd2_journal_get_write_access() /
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jbd2_journal_get_undo_access() as appropriate, this allows the
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journalling layer to copy the unmodified
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data if it needs to. After all the buffer may be part of a previously
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uncommitted transaction. At this point you are at last ready to modify a
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buffer, and once you are have done so you need to call
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:c:func:`jbd2_journal_dirty_metadata`. Or if you've asked for access to a
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jbd2_journal_dirty_metadata(). Or if you've asked for access to a
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buffer you now know is now longer required to be pushed back on the
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device you can call :c:func:`jbd2_journal_forget` in much the same way as you
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might have used :c:func:`bforget` in the past.
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device you can call jbd2_journal_forget() in much the same way as you
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might have used bforget() in the past.
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A :c:func:`jbd2_journal_flush` may be called at any time to commit and
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A jbd2_journal_flush() may be called at any time to commit and
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checkpoint all your transactions.
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Then at umount time , in your :c:func:`put_super` you can then call
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:c:func:`jbd2_journal_destroy` to clean up your in-core journal object.
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Then at umount time , in your put_super() you can then call
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jbd2_journal_destroy() to clean up your in-core journal object.
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Unfortunately there a couple of ways the journal layer can cause a
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deadlock. The first thing to note is that each task can only have a
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single outstanding transaction at any one time, remember nothing commits
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until the outermost :c:func:`jbd2_journal_stop`. This means you must complete
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until the outermost jbd2_journal_stop(). This means you must complete
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the transaction at the end of each file/inode/address etc. operation you
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perform, so that the journalling system isn't re-entered on another
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journal. Since transactions can't be nested/batched across differing
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journals, and another filesystem other than yours (say ext4) may be
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modified in a later syscall.
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The second case to bear in mind is that :c:func:`jbd2_journal_start` can block
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The second case to bear in mind is that jbd2_journal_start() can block
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if there isn't enough space in the journal for your transaction (based
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on the passed nblocks param) - when it blocks it merely(!) needs to wait
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for transactions to complete and be committed from other tasks, so
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essentially we are waiting for :c:func:`jbd2_journal_stop`. So to avoid
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deadlocks you must treat :c:func:`jbd2_journal_start` /
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:c:func:`jbd2_journal_stop` as if they were semaphores and include them in
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essentially we are waiting for jbd2_journal_stop(). So to avoid
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deadlocks you must treat jbd2_journal_start() /
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jbd2_journal_stop() as if they were semaphores and include them in
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your semaphore ordering rules to prevent
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deadlocks. Note that :c:func:`jbd2_journal_extend` has similar blocking
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behaviour to :c:func:`jbd2_journal_start` so you can deadlock here just as
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easily as on :c:func:`jbd2_journal_start`.
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deadlocks. Note that jbd2_journal_extend() has similar blocking
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behaviour to jbd2_journal_start() so you can deadlock here just as
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easily as on jbd2_journal_start().
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Try to reserve the right number of blocks the first time. ;-). This will
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be the maximum number of blocks you are going to touch in this
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@ -116,8 +116,8 @@ called after each transaction commit. You can also use
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that need processing when the transaction commits.
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JBD2 also provides a way to block all transaction updates via
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:c:func:`jbd2_journal_lock_updates()` /
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:c:func:`jbd2_journal_unlock_updates()`. Ext4 uses this when it wants a
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jbd2_journal_lock_updates() /
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jbd2_journal_unlock_updates(). Ext4 uses this when it wants a
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window with a clean and stable fs for a moment. E.g.
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::
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