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![Eric Biggers](/assets/img/avatar_default.png)
Perhaps long overdue, add a documentation file for filesystem-level encryption, a.k.a. fscrypt or fs/crypto/, to the Documentation directory. The new file is based loosely on the latest version of the "EXT4 Encryption Design Document (public version)" Google Doc, but with many improvements made, including: - Reflect the reality that it is not specific to ext4 anymore. - More thoroughly document the design and user-visible API/behavior. - Replace outdated information, such as the outdated explanation of how encrypted filenames are hashed for indexed directories and how encrypted filenames are presented to userspace without the key. (This was changed just before release.) For now the focus is on the design and user-visible API/behavior, not on how to add encryption support to a filesystem --- since the internal API is still pretty messy and any standalone documentation for it would become outdated as things get refactored over time. Reviewed-by: Michael Halcrow <mhalcrow@google.com> Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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=====================
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Linux Filesystems API
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=====================
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The Linux VFS
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=============
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The Filesystem types
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--------------------
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.. kernel-doc:: include/linux/fs.h
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:internal:
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The Directory Cache
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-------------------
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.. kernel-doc:: fs/dcache.c
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:export:
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.. kernel-doc:: include/linux/dcache.h
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:internal:
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Inode Handling
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--------------
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.. kernel-doc:: fs/inode.c
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:export:
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.. kernel-doc:: fs/bad_inode.c
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:export:
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Registration and Superblocks
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----------------------------
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.. kernel-doc:: fs/super.c
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:export:
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File Locks
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----------
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.. kernel-doc:: fs/locks.c
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:export:
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.. kernel-doc:: fs/locks.c
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:internal:
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Other Functions
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---------------
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.. kernel-doc:: fs/mpage.c
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:export:
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.. kernel-doc:: fs/namei.c
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:export:
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.. kernel-doc:: fs/buffer.c
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:export:
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.. kernel-doc:: block/bio.c
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:export:
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.. kernel-doc:: fs/seq_file.c
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:export:
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.. kernel-doc:: fs/filesystems.c
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:export:
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.. kernel-doc:: fs/fs-writeback.c
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:export:
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.. kernel-doc:: fs/block_dev.c
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:export:
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The proc filesystem
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===================
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sysctl interface
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----------------
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.. kernel-doc:: kernel/sysctl.c
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:export:
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proc filesystem interface
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-------------------------
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.. kernel-doc:: fs/proc/base.c
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:internal:
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Events based on file descriptors
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================================
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.. kernel-doc:: fs/eventfd.c
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:export:
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The Filesystem for Exporting Kernel Objects
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===========================================
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.. kernel-doc:: fs/sysfs/file.c
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:export:
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.. kernel-doc:: fs/sysfs/symlink.c
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:export:
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The debugfs filesystem
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======================
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debugfs interface
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-----------------
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.. kernel-doc:: fs/debugfs/inode.c
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:export:
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.. kernel-doc:: fs/debugfs/file.c
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:export:
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The Linux Journalling API
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=========================
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Overview
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--------
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Details
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~~~~~~~
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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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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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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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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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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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Now you can go ahead and start modifying the underlying filesystem.
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Almost.
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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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:c:func:`jbd2_journal_start` and its counterpart :c:func:`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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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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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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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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A :c:func:`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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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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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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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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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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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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transaction. I advise having a look at at least ext4_jbd.h to see the
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basis on which ext4 uses to make these decisions.
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Another wriggle to watch out for is your on-disk block allocation
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strategy. Why? Because, if you do a delete, you need to ensure you
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haven't reused any of the freed blocks until the transaction freeing
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these blocks commits. If you reused these blocks and crash happens,
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there is no way to restore the contents of the reallocated blocks at the
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end of the last fully committed transaction. One simple way of doing
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this is to mark blocks as free in internal in-memory block allocation
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structures only after the transaction freeing them commits. Ext4 uses
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journal commit callback for this purpose.
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With journal commit callbacks you can ask the journalling layer to call
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a callback function when the transaction is finally committed to disk,
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so that you can do some of your own management. You ask the journalling
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layer for calling the callback by simply setting
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``journal->j_commit_callback`` function pointer and that function is
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called after each transaction commit. You can also use
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``transaction->t_private_list`` for attaching entries to a transaction
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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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window with a clean and stable fs for a moment. E.g.
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::
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jbd2_journal_lock_updates() //stop new stuff happening..
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jbd2_journal_flush() // checkpoint everything.
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..do stuff on stable fs
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jbd2_journal_unlock_updates() // carry on with filesystem use.
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The opportunities for abuse and DOS attacks with this should be obvious,
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if you allow unprivileged userspace to trigger codepaths containing
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these calls.
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Summary
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~~~~~~~
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Using the journal is a matter of wrapping the different context changes,
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being each mount, each modification (transaction) and each changed
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buffer to tell the journalling layer about them.
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Data Types
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----------
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The journalling layer uses typedefs to 'hide' the concrete definitions
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of the structures used. As a client of the JBD2 layer you can just rely
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on the using the pointer as a magic cookie of some sort. Obviously the
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hiding is not enforced as this is 'C'.
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Structures
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~~~~~~~~~~
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.. kernel-doc:: include/linux/jbd2.h
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:internal:
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Functions
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---------
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The functions here are split into two groups those that affect a journal
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as a whole, and those which are used to manage transactions
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Journal Level
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~~~~~~~~~~~~~
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.. kernel-doc:: fs/jbd2/journal.c
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:export:
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.. kernel-doc:: fs/jbd2/recovery.c
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:internal:
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Transasction Level
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~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: fs/jbd2/transaction.c
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See also
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--------
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`Journaling the Linux ext2fs Filesystem, LinuxExpo 98, Stephen
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Tweedie <http://kernel.org/pub/linux/kernel/people/sct/ext3/journal-design.ps.gz>`__
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`Ext3 Journalling FileSystem, OLS 2000, Dr. Stephen
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Tweedie <http://olstrans.sourceforge.net/release/OLS2000-ext3/OLS2000-ext3.html>`__
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splice API
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==========
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splice is a method for moving blocks of data around inside the kernel,
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without continually transferring them between the kernel and user space.
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.. kernel-doc:: fs/splice.c
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pipes API
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=========
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Pipe interfaces are all for in-kernel (builtin image) use. They are not
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exported for use by modules.
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.. kernel-doc:: include/linux/pipe_fs_i.h
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:internal:
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.. kernel-doc:: fs/pipe.c
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Encryption API
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==============
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A library which filesystems can hook into to support transparent
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encryption of files and directories.
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.. toctree::
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:maxdepth: 2
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fscrypt
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