[PATCH] jbd2: don't write superblock when unmounting an ro filesystem

[Eric Sandeen <sandeen@redhat.com>]

This sequence:

# truncate --size=65536 fsfile
# losetup --offset 65536 /dev/loop0 fsfile 
# mkfs.ext4 /dev/loop0 
# losetup -d /dev/loop0
# mount -o loop,ro,offset=65536 fsfile  mnt/
# umount mnt
# dmesg | tail

results in an IO error when unmounting the RO filesystem:

[  312.386074] SELinux: initialized (dev loop1, type ext4), uses xattr
[  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 behavior changed with:

commit 24bcc89c7e7c64982e6192b4952a0a92379fc341
Author: Jan Kara <jack@suse.cz>
Date:   Tue Mar 13 15:41:04 2012 -0400

    jbd2: split updating of journal superblock and marking journal empty

which lost some of the magic in jbd2_journal_update_superblock() which
used to test for a journal with no outstanding transactions.

I'm not sure if the following is quite the right approach, but it fixes
it for me.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

p.s. no idea why this only happens if I use a loop device with an offset!

diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
index e9a3c4c..987ec76 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);