[RFC PATCH 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 simply re-enqueuing the inode from the MAIN
into the STAGING queue.

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 | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
index 87c009e0c59a..337b5289cf11 100644
--- a/fs/ext4/fast_commit.c
+++ b/fs/ext4/fast_commit.c
@@ -396,12 +396,19 @@ static int ext4_fc_track_template(
 		return ret;
 
 	spin_lock(&sbi->s_fc_lock);
-	if (list_empty(&EXT4_I(inode)->i_fc_list))
-		list_add_tail(&EXT4_I(inode)->i_fc_list,
-				(sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
-				 sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING) ?
-				&sbi->s_fc_q[FC_Q_STAGING] :
-				&sbi->s_fc_q[FC_Q_MAIN]);
+	if (sbi->s_journal->j_flags & JBD2_FULL_COMMIT_ONGOING ||
+	    sbi->s_journal->j_flags & JBD2_FAST_COMMIT_ONGOING) {
+		if (list_empty(&EXT4_I(inode)->i_fc_list))
+			list_add_tail(&EXT4_I(inode)->i_fc_list,
+				      &sbi->s_fc_q[FC_Q_STAGING]);
+		else
+			list_move_tail(&EXT4_I(inode)->i_fc_list,
+				       &sbi->s_fc_q[FC_Q_STAGING]);
+	} else {
+		if (list_empty(&EXT4_I(inode)->i_fc_list))
+			list_add_tail(&EXT4_I(inode)->i_fc_list,
+				      &sbi->s_fc_q[FC_Q_MAIN]);
+	}
 	spin_unlock(&sbi->s_fc_lock);
 
 	return ret;