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ext4: update code documentation

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This patch updates code documentation to reflect the commit path changes
made in this series.

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>

code docs

Link: https://patch.msgid.link/20250508175908.1004880-7-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Harshad Shirwadkar
2025-05-08 17:59:05 +00:00
committed by Theodore Ts'o
parent ed45d33113
commit 69f35ca189
1 changed files with 31 additions and 17 deletions
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48
fs/ext4/fast_commit.c
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@@ -50,19 +50,27 @@
* that need to be committed during a fast commit in another in memory queue of
* inodes. During the commit operation, we commit in the following order:
*
* [1] Lock inodes for any further data updates by setting COMMITTING state
* [2] Submit data buffers of all the inodes
* [3] Wait for [2] to complete
* [4] Commit all the directory entry updates in the fast commit space
* [5] Commit all the changed inode structures
* [6] Write tail tag (this tag ensures the atomicity, please read the following
* [1] Prepare all the inodes to write out their data by setting
* "EXT4_STATE_FC_FLUSHING_DATA". This ensures that inode cannot be
* deleted while it is being flushed.
* [2] Flush data buffers to disk and clear "EXT4_STATE_FC_FLUSHING_DATA"
* state.
* [3] Lock the journal by calling jbd2_journal_lock_updates. This ensures that
* all the exsiting handles finish and no new handles can start.
* [4] Mark all the fast commit eligible inodes as undergoing fast commit
* by setting "EXT4_STATE_FC_COMMITTING" state.
* [5] Unlock the journal by calling jbd2_journal_unlock_updates. This allows
* starting of new handles. If new handles try to start an update on
* any of the inodes that are being committed, ext4_fc_track_inode()
* will block until those inodes have finished the fast commit.
* [6] Commit all the directory entry updates in the fast commit space.
* [7] Commit all the changed inodes in the fast commit space and clear
* "EXT4_STATE_FC_COMMITTING" for these inodes.
* [8] Write tail tag (this tag ensures the atomicity, please read the following
* section for more details).
* [7] Wait for [4], [5] and [6] to complete.
*
* All the inode updates must call ext4_fc_start_update() before starting an
* update. If such an ongoing update is present, fast commit waits for it to
* complete. The completion of such an update is marked by
* ext4_fc_stop_update().
* All the inode updates must be enclosed within jbd2_jounrnal_start()
* and jbd2_journal_stop() similar to JBD2 journaling.
*
* Fast Commit Ineligibility
* -------------------------
@@ -143,6 +151,13 @@
* similarly. Thus, by converting a non-idempotent procedure into a series of
* idempotent outcomes, fast commits ensured idempotence during the replay.
*
* Locking
* -------
* sbi->s_fc_lock protects the fast commit inodes queue and the fast commit
* dentry queue. ei->i_fc_lock protects the fast commit related info in a given
* inode. Most of the code avoids acquiring both the locks, but if one must do
* that then sbi->s_fc_lock must be acquired before ei->i_fc_lock.
*
* TODOs
* -----
*
@@ -157,13 +172,12 @@
* fast commit recovery even if that area is invalidated by later full
* commits.
*
* 1) Fast commit's commit path locks the entire file system during fast
* commit. This has significant performance penalty. Instead of that, we
* should use ext4_fc_start/stop_update functions to start inode level
* updates from ext4_journal_start/stop. Once we do that we can drop file
* system locking during commit path.
* 1) Handle more ineligible cases.
*
* 2) Handle more ineligible cases.
* 2) Change ext4_fc_commit() to lookup logical to physical mapping using extent
* status tree. This would get rid of the need to call ext4_fc_track_inode()
* before acquiring i_data_sem. To do that we would need to ensure that
* modified extents from the extent status tree are not evicted from memory.
*/
#include <trace/events/ext4.h>