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

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Date:	Wed, 24 Feb 2010 18:05:06 +0100
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Dmitry Monakhov <dmonakhov@...nvz.org>, 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

On Wed 24-02-10 10:57:59, Eric Sandeen wrote:
> 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...
  Well, my opinion is that VFS should take care about the rw->ro transition
so that it isn't racy...

> 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.
  With freeze you can still keep dirty data in cache until the filesystem
unfreezes so it's a different situation from rw->ro transition.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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