From: Eric Sandeen Subject: [PATCH] jbd2: don't write superblock when unmounting an ro filesystem Date: Tue, 24 Jul 2012 17:05:28 -0500 Message-ID: <500F1C28.8010800@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Jan Kara To: ext4 development Return-path: Received: from mx1.redhat.com ([209.132.183.28]:54468 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754309Ab2GXWFc (ORCPT ); Tue, 24 Jul 2012 18:05:32 -0400 Sender: linux-ext4-owner@vger.kernel.org List-ID: 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 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 --- 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);