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

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Date:	Tue, 18 Dec 2012 09:20:02 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Michael Tokarev <mjt@....msk.ru>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: do not try to write superblock on journal-less
 readonly remount

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 ;)

But Lukas' question may be what's holding Ted up.

-Eric

> 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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