From: Jan Kara Subject: Re: [ext3] Changes to block device after an ext3 mount point has been remounted readonly Date: Wed, 24 Feb 2010 18:05:06 +0100 Message-ID: <20100224170506.GN3687@quack.suse.cz> References: <9F53CAF8-B6B4-40EB-89FA-CD6779D17DBE@sun.com> <20100222223252.GA13882@atrey.karlin.mff.cuni.cz> <20100222230552.GB13882@atrey.karlin.mff.cuni.cz> <16F918FB-F45D-478E-9358-550BB39E277E@sun.com> <20100223135531.GA7699@atrey.karlin.mff.cuni.cz> <877hq2tyg8.fsf@openvz.org> <4B855A97.4010702@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dmitry Monakhov , Jan Kara , Camille Moncelier , "linux-fsdevel@vger.kernel.org" , ext4 development To: Eric Sandeen Return-path: Received: from cantor.suse.de ([195.135.220.2]:51065 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757298Ab0BXRE7 (ORCPT ); Wed, 24 Feb 2010 12:04:59 -0500 Content-Disposition: inline In-Reply-To: <4B855A97.4010702@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed 24-02-10 10:57:59, Eric Sandeen wrote: > Dmitry Monakhov wrote: > > Jan Kara 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 SUSE Labs, CR