linux-ext4 - [PATCH v5] ext4: fix fast commit inode enqueueing during a full journal commit

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Message-ID: <20240717172220.14201-1-luis.henriques@linux.dev>
Date: Wed, 17 Jul 2024 18:22:20 +0100
From: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
To: Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger@...ger.ca>,
	Jan Kara <jack@...e.cz>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>
Cc: linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Subject: [PATCH v5] ext4: fix fast commit inode enqueueing during a full journal commit

When a full journal commit is on-going, any fast commit has to be enqueued
into a different queue: FC_Q_STAGING instead of FC_Q_MAIN.  This enqueueing
is done only once, i.e. if an inode is already queued in a previous fast
commit entry it won't be enqueued again.  However, if a full commit starts
_after_ the inode is enqueued into FC_Q_MAIN, the next fast commit needs to
be done into FC_Q_STAGING.  And this is not being done in function
ext4_fc_track_template().

This patch fixes the issue by re-enqueuing an inode into the STAGING queue
during the fast commit clean-up callback when doing a full commit.  However,
to prevent a race with a fast-commit, the clean-up callback has to be called
with the journal locked.

This bug was found using fstest generic/047.  This test creates several 32k
bytes files, sync'ing each of them after it's creation, and then shutting
down the filesystem.  Some data may be loss in this operation; for example a
file may have it's size truncated to zero.

Suggested-by: Jan Kara <jack@...e.cz>
Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>
---
Hi!

And here's another attempt to fix this bug.  The most significant change is
that now it doesn't assume a 'special' meaning for a tid of '0'.  Which is
a wrong assumption as Jan has shown.

I've also added a Suggested-by: tag, although Jan pretty much owns this
patch -- I have simply tested it and sent it out!

 fs/ext4/fast_commit.c | 15 ++++++++++++++-
 fs/jbd2/journal.c     |  2 +-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 3926a05eceee..df71fd5b1fed 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -1288,8 +1288,21 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
 		list_del_init(&iter->i_fc_list);
 		ext4_clear_inode_state(&iter->vfs_inode,
 				       EXT4_STATE_FC_COMMITTING);
-		if (tid_geq(tid, iter->i_sync_tid))
+		if (tid_geq(tid, iter->i_sync_tid)) {
 			ext4_fc_reset_inode(&iter->vfs_inode);
+		} else if (full) {
+			/*
+			 * We are called after a full commit, inode has been
+			 * modified while the commit was running. Re-enqueue
+			 * the inode into STAGING, which will then be splice
+			 * back into MAIN. This cannot happen during
+			 * fastcommit because the journal is locked all the
+			 * time in that case (and tid doesn't increase so
+			 * tid check above isn't reliable).
+			 */
+			list_add_tail(&EXT4_I(&iter->vfs_inode)->i_fc_list,
+				      &sbi->s_fc_q[FC_Q_STAGING]);
+		}
 		/* Make sure EXT4_STATE_FC_COMMITTING bit is clear */
 		smp_mb();
 #if (BITS_PER_LONG < 64)
diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index 1ebf2393bfb7..291a431f8aaf 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -740,9 +740,9 @@ EXPORT_SYMBOL(jbd2_fc_begin_commit);
  */
 static int __jbd2_fc_end_commit(journal_t *journal, tid_t tid, bool fallback)
 {
-	jbd2_journal_unlock_updates(journal);
 	if (journal->j_fc_cleanup_callback)
 		journal->j_fc_cleanup_callback(journal, 0, tid);
+	jbd2_journal_unlock_updates(journal);
 	write_lock(&journal->j_state_lock);
 	journal->j_flags &= ~JBD2_FAST_COMMIT_ONGOING;
 	if (fallback)