linux-ext4 - Re: [PATCH] ext4: do not try to write superblock on journal-less readonly remount

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Date:	Thu, 20 Dec 2012 13:24:41 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: do not try to write superblock on journal-less
 readonly remount

On 18.12.2012 19:20, Eric Sandeen wrote:
> On 12/18/12 2:14 AM, Michael Tokarev wrote:
>> Ping?  Almost 2 months has passed since initial patch...
>>
>> Thanks,
>>
>> /mjt
> 
> Michael, Lukas commented a while ago (10/25) that he was unable to reproduce
> the problem.  Do you have any comment on that?  TBH it's long enough
> ago that I've forgotten the issue ;)

Yeah okay.

The two reproducers I've found so far are both about using true read-only
media.  One original where I've hit it was a virtual machine (KVM) with
a read-only virtio drive:

  kvm ... -drive file=guest.img,if=virtio,readonly=yes

(It does not work with IDE emulation because there's no way on IDE to pass
the "readonly" flag).

Another way I found is to use an SD card in an USB card reader with the
"read-only" jumper in "on" position (or a micro-SD to SD adaptor with
such a jumper).

In both cases mount -o remount in guest results in a series of error
messages from kernel - it complains about write errors.

My initial comment that it is enough to set block device to be read-only
using blockdev --setro is wrong, -- apparently ext4fs uses write paths
that bypasses the block-level RO checks -- which is, apparenlty, also
wrong, but it's a different matter.

Thanks,

/mjt

>> On 25.10.2012 12:39, Michael Tokarev wrote:
>>> 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.
>>>
>>> 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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