[PATCH v4] ext4: fix fast commit inode enqueueing during a full journal commit

["Luis Henriques (SUSE)" <luis.henriques@linux.dev>]

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 if it has a tid (i_sync_tid)
greater than the one being handled.  The STAGING queue will then be spliced
back into MAIN.

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.

Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
---
Hi!

v4 of this patch enqueues the inode into STAGING *only* if the current tid
is non-zero.  It will be zero when doing an fc commit, and this would mean
to always re-enqueue the inode.  This fixes the regressions caught by Ted
in v3 with fstests generic/472 generic/496 generic/643.

Also, since 2nd patch of v3 has already been merged, I've rebased this patch
to be applied on top of it.

 fs/ext4/fast_commit.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 3926a05eceee..facbc8dbbaa2 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -1290,6 +1290,16 @@ static void ext4_fc_cleanup(journal_t *journal, int full, tid_t tid)
 				       EXT4_STATE_FC_COMMITTING);
 		if (tid_geq(tid, iter->i_sync_tid))
 			ext4_fc_reset_inode(&iter->vfs_inode);
+		} else if (tid) {
+			/*
+			 * If the tid is valid (i.e. non-zero) re-enqueue the
+			 * inode into STAGING, which will then be splice back
+			 * into MAIN
+			 */
+			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)