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authorLinus Torvalds <torvalds@linux-foundation.org>2020-12-24 14:16:02 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-12-24 14:16:02 -0800
commit555a6e8c11e6282bb2704ef1cee64ceaeb41773e (patch)
treeaab2e492fcfdbd96bea97e409919e8979bfc6525 /Documentation
parent2f2fce3d535779cb1b0d77ce839029d5d875d4f4 (diff)
parentbe993933d2e997fdb72b8b1418d2a84df79b8962 (diff)
Merge tag 'ext4_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4
Pull ext4 updates from Ted Ts'o: "Various bug fixes and cleanups for ext4; no new features this cycle" * tag 'ext4_for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tytso/ext4: (29 commits) ext4: remove unnecessary wbc parameter from ext4_bio_write_page ext4: avoid s_mb_prefetch to be zero in individual scenarios ext4: defer saving error info from atomic context ext4: simplify ext4 error translation ext4: move functions in super.c ext4: make ext4_abort() use __ext4_error() ext4: standardize error message in ext4_protect_reserved_inode() ext4: remove redundant sb checksum recomputation ext4: don't remount read-only with errors=continue on reboot ext4: fix deadlock with fs freezing and EA inodes jbd2: add a helper to find out number of fast commit blocks ext4: make fast_commit.h byte identical with e2fsprogs/fast_commit.h ext4: fix fall-through warnings for Clang ext4: add docs about fast commit idempotence ext4: remove the unused EXT4_CURRENT_REV macro ext4: fix an IS_ERR() vs NULL check ext4: check for invalid block size early when mounting a file system ext4: fix a memory leak of ext4_free_data ext4: delete nonsensical (commented-out) code inside ext4_xattr_block_set() ext4: update ext4_data_block_valid related comments ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/ext4/journal.rst50
1 files changed, 50 insertions, 0 deletions
diff --git a/Documentation/filesystems/ext4/journal.rst b/Documentation/filesystems/ext4/journal.rst
index 849d5b119eb8..cdbfec473167 100644
--- a/Documentation/filesystems/ext4/journal.rst
+++ b/Documentation/filesystems/ext4/journal.rst
@@ -681,3 +681,53 @@ Here is the list of supported tags and their meanings:
- Stores the TID of the commit, CRC of the fast commit of which this tag
represents the end of
+Fast Commit Replay Idempotence
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Fast commits tags are idempotent in nature provided the recovery code follows
+certain rules. The guiding principle that the commit path follows while
+committing is that it stores the result of a particular operation instead of
+storing the procedure.
+
+Let's consider this rename operation: 'mv /a /b'. Let's assume dirent '/a'
+was associated with inode 10. During fast commit, instead of storing this
+operation as a procedure "rename a to b", we store the resulting file system
+state as a "series" of outcomes:
+
+- Link dirent b to inode 10
+- Unlink dirent a
+- Inode 10 with valid refcount
+
+Now when recovery code runs, it needs "enforce" this state on the file
+system. This is what guarantees idempotence of fast commit replay.
+
+Let's take an example of a procedure that is not idempotent and see how fast
+commits make it idempotent. Consider following sequence of operations:
+
+1) rm A
+2) mv B A
+3) read A
+
+If we store this sequence of operations as is then the replay is not idempotent.
+Let's say while in replay, we crash after (2). During the second replay,
+file A (which was actually created as a result of "mv B A" operation) would get
+deleted. Thus, file named A would be absent when we try to read A. So, this
+sequence of operations is not idempotent. However, as mentioned above, instead
+of storing the procedure fast commits store the outcome of each procedure. Thus
+the fast commit log for above procedure would be as follows:
+
+(Let's assume dirent A was linked to inode 10 and dirent B was linked to
+inode 11 before the replay)
+
+1) Unlink A
+2) Link A to inode 11
+3) Unlink B
+4) Inode 11
+
+If we crash after (3) we will have file A linked to inode 11. During the second
+replay, we will remove file A (inode 11). But we will create it back and make
+it point to inode 11. We won't find B, so we'll just skip that step. At this
+point, the refcount for inode 11 is not reliable, but that gets fixed by the
+replay of last inode 11 tag. Thus, by converting a non-idempotent procedure
+into a series of idempotent outcomes, fast commits ensured idempotence during
+the replay.