[PATCH v3 1/2] 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>
---
 fs/ext4/fast_commit.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 87c009e0c59a..088bd509b116 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -1282,8 +1282,17 @@ 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 (iter->i_sync_tid <= tid)
+		if (iter->i_sync_tid <= tid) {
 			ext4_fc_reset_inode(&iter->vfs_inode);
+		} else {
+			/*
+			 * re-enqueue inode into STAGING, which later will 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)