jbd2: don't write superblock when if its empty

This sequence:

# truncate --size=1g fsfile
# mkfs.ext4 -F fsfile
# mount -o loop,ro fsfile /mnt
# umount /mnt
# dmesg | tail

results in an IO error when unmounting the RO filesystem:

[  318.020828] Buffer I/O error on device loop1, logical block 196608
[  318.027024] lost page write due to I/O error on loop1
[  318.032088] JBD2: Error -5 detected when updating journal superblock for loop1-8.

This was a regression introduced by commit 24bcc89c7e7c: "jbd2: split
updating of journal superblock and marking journal empty".

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index bd23f2ebaa67..0f16edd51f67 100644
--- a/fs/jbd2/journal.c
+++ b/fs/jbd2/journal.c
@@ -1354,6 +1354,11 @@ static void jbd2_mark_journal_empty(journal_t *journal)
 
         BUG_ON(!mutex_is_locked(&journal->j_checkpoint_mutex));
         read_lock(&journal->j_state_lock);
+        /* Is it already empty? */
+        if (sb->s_start == 0) {
+                read_unlock(&journal->j_state_lock);
+                return;
+        }
         jbd_debug(1, "JBD2: Marking journal as empty (seq %d)\n",
                   journal->j_tail_sequence);