linux-ext4 - Re: [PATCH] jbd2: don't write superblock when unmounting an ro filesystem

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Date:	Wed, 15 Aug 2012 14:14:03 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] jbd2: don't write superblock when unmounting an ro
 filesystem

On Tue 24-07-12 17:05:28, 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.
  Ted, did this patch fall through cracks? I've ported the fix to JBD and
added it to my tree. I plan to send it to Linus in a few days. Also I've
CC'd stable since this is a bit annoying regression.

								Honza

> 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!
> 
> 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);
>  
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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