linux-ext4 - Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly

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Message-ID: <4B855A97.4010702@redhat.com>
Date:	Wed, 24 Feb 2010 10:57:59 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
CC:	Jan Kara <jack@...e.cz>, Camille Moncelier <pix@...life.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [ext3] Changes to block device after an ext3 mount point has
 been remounted readonly

Dmitry Monakhov wrote:
> Jan Kara <jack@...e.cz> writes:
> 
>>> The fact is that I've been able to reproduce the problem on LVM block
>>> devices, and sd* block devices so it's definitely not a loop device
>>> specific problem.
>>>
>>> By the way, I tried several other things other than "echo s
>>>> /proc/sysrq_trigger" I tried multiple sync followed with a one minute
>>> "sleep",
>>>
>>> "echo 3 >/proc/sys/vm/drop_caches" seems to lower the chances of "hash
>>> changes" but doesn't stops them.
>>   Strange. When I use sync(1) in your script and use /dev/sda5 instead of a
>> /dev/loop0, I cannot reproduce the problem (was running the script for
>> something like an hour).
> Theoretically some pages may exist after rw=>ro remount
> because of generic race between write/sync, And they will be written
> in by writepage if page already has buffers. This not happen in ext4
> because. Each time it try to perform writepages it try to start_journal
> and this result in EROFS.
> The race bug will be closed some day but new one may appear again.
> 
> Let's be honest and change ext3 writepage like follows:
> - check ROFS flag inside write page
> - dump writepage's errors.
> 
> 

sounds like the wrong approach to me, we really need to fix the root
cause and make remount,ro finish the job, I think.

Throwing away writes which an application already thinks are completed
just because remount,ro didn't keep up sounds like a bad idea.  I think
I would much rather have the write complete shortly after the readonly
transition, if I had to choose...

I haven't looked at these paths at all but just hand-wavily,
remount,ro should follow pretty much the same path as freeze,
I think.  And if freeze isn't getting everything on-disk we have
an even bigger problem.

-Eric
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