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Date:	Tue, 24 Jul 2012 17:18:42 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	ext4 development <linux-ext4@...r.kernel.org>
CC:	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] jbd2: don't write superblock when unmounting an ro filesystem

On 7/24/12 5:05 PM, Eric Sandeen wrote:
> 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@...e.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@...hat.com>
> ---
> 
> p.s. no idea why this only happens if I use a loop device with an offset!

PEBKAC.  no-offset ro loop mount does the same.

-Eric

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