Date:	Thu, 25 Oct 2012 14:43:09 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Michael Tokarev <mjt@....msk.ru>
cc:	linux-ext4@...r.kernel.org, sandeen@...hat.com
Subject: Re: [PATCH] ext4: do not try to write superblock on journal-less
 readonly remount

On Thu, 25 Oct 2012, Michael Tokarev wrote:

> Date: Thu, 25 Oct 2012 12:39:57 +0400
> From: Michael Tokarev <mjt@....msk.ru>
> To: linux-ext4@...r.kernel.org
> Cc: sandeen@...hat.com, Michael Tokarev <mjt@....msk.ru>
> Subject: [PATCH] ext4: do not try to write superblock on journal-less readonly
>      remount
> 
> When a journal-less ext4 filesystem is mounted on a read-only block
> device (blockdev --setro will do), each remount (for other, unrelated,
> flags, like suid=>nosuid etc) results in a series of scary messages
> from kernel telling about I/O errors on the device.

Hi Michael,

I am not able to reproduce the problem you're seeing:

mkfs.ext4 /dev/sdd1
tune2fs -O ^has_journal /dev/sdd1
blockdev --setro /dev/sdd1
mount /dev/sdd1 /mnt/test

and then

mount -o remount,suid /dev/sdd1
mount -o remount,nosuid /dev/sdd1
mount -o remount,noatime /dev/sdd1
mount -o remount,relatime /dev/sdd1
mount -o remount,relatime,commit=20 /dev/sdd1

just does not produce any errors. Both /var/log/messages and dmesg
are clear.

mount shows
...
/dev/sdd1 on /mnt/test type ext4 (ro,nosuid,noatime,relatime,commit=20)
...


This is on 3.7.0-rc2

Am I missing something ?

Thanks!
-Lukas

> 
> This is becauese of the following code ext4_remount():
> 
>        if (sbi->s_journal == NULL)
>                 ext4_commit_super(sb, 1);
> 
> at the end of remount procedure, which forces writing (flushing) of
> a superblock regardless whenever it is dirty or not, if the filesystem
> is readonly or not, and whenever the device itself is readonly or not.
> 
> The proposed fix tests whenever both old mount flags and new mount
> flags does not include MS_READONLY, and only in this case calls
> ext4_commit_super().
> 
> Maybe it is sufficient to check for MS_READONLY just in old mount
> options (old_sb_flags).  Note this is journal-less mode, so, for
> example, we weren't have journal replay operation, so if old flags
> include MS_REASONLY, we shuold have no dirty blocks at all, and
> there's no reason to call ext4_commit_super().
> 
> But only in case both old and new flags include MS_READONLY we're
> certain we will not write anything - if new flag does not include
> this bit, we will write sooner or later anyway, so preventing just
> one commit_super() at the _beginning_ of mount is not really necessary.
> 
> This change probably applicable to -stable, -- not because it fixes
> a serious bug, but because the messages printed by the kernel are
> rather scary for an average user.  On the other hand, actual usage
> of ext4 in nojournal mode on a read-only medium is very rare.
> 
> Thanks to Eric Sandeen for help in diagnosing this issue.
> 
> Signed-off-By: Michael Tokarev <mjt@....msk.ru>
> ---
>  fs/ext4/super.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3e0851e..2e896fd 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4687,7 +4687,7 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
>  	}
>  
>  	ext4_setup_system_zone(sb);
> -	if (sbi->s_journal == NULL)
> +	if (sbi->s_journal == NULL && !(sb->s_flags & old_sb_flags & MS_RDONLY))
>  		ext4_commit_super(sb, 1);
>  
>  	unlock_super(sb);
> 
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