From: Jan Kara Subject: Re: [RFC PATCH] mark buffer_head mapping preallocate area as new during write_begin with delayed allocation Date: Wed, 29 Apr 2009 20:13:21 +0200 Message-ID: <20090429181321.GA22936@duck.suse.cz> References: <1240873494.6775.8.camel@mingming-laptop> <20090428042049.GA6520@skywalker> <20090428093145.GA13719@skywalker> <20090428124821.GJ22104@mit.edu> <20090428163554.GA27670@skywalker> <20090428170047.GC24043@mit.edu> <20090428185749.GA3275@skywalker> <49F75A9C.6000307@redhat.com> <20090429115727.GC18195@atrey.karlin.mff.cuni.cz> <49F85F45.1020805@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , "Aneesh Kumar K.V" , Theodore Tso , Mingming Cao , linux-ext4@vger.kernel.org To: Eric Sandeen Return-path: Received: from cantor2.suse.de ([195.135.220.15]:55391 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751465AbZD2SNX (ORCPT ); Wed, 29 Apr 2009 14:13:23 -0400 Content-Disposition: inline In-Reply-To: <49F85F45.1020805@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed 29-04-09 09:08:05, Eric Sandeen wrote: > Jan Kara wrote: > >> Aneesh Kumar K.V wrote: > >>> On Tue, Apr 28, 2009 at 01:00:47PM -0400, Theodore Tso wrote: > >>>> On Tue, Apr 28, 2009 at 10:05:54PM +0530, Aneesh Kumar K.V wrote: > >> ... > >>>>>> The other problem seems to be in the case of a delayed allocation > >>>>>> write, where we return a buffer_head which is marked new, and this > >>>>>> causes block_prepare_write() to call unmap_underlying_metadata(dev, 0). > >>>>> Not just that. On block allocation we are not calling > >>>>> unmap_underlying_metadata(dev, blocknumber) for delayed allocated > >>>>> blocks. That would imply file corruption. > >>>> I don't think I'm following you . If we write into block that was > >>>> delayed allocated. Are you saying we might get in trouble of the > >>>> delayed allocation block is mmap'ed in? > >>> We allocate blocks for delayed buffer during writepage. Now we need to > >>> make sure after getting the blocks we drop the old buffer_head mapping > >>> that we may have with this particular block attached to the block > >>> device. That is done by calling unmap_underlying_metadata. Now the > >>> current code doesn't call unmap_underlying_metadata for delayed > >>> allocated blocks. That would mean we can see corrupt files if old > >>> buffer_head mapping gets synced to disk AFTER we write the new > >>> buffer_head mapping. > >> > >> Talking w/ Aneesh on IRC, I don't see how we can have stray dirty > >> mappings lying around for this block device unless someone is writing > >> directly to the mounted block device, which I don't think is ever > >> considered safe ... > >> > >> I'm not quite sure what the call to __unmap_underlying_blocks() in > >> mpage_da_map_blocks() is for, I guess? > > For ext3 / ext4 I think we don't need unmap_underlying_blocks() since > > before we reallocate a block, we make sure that the transaction freeing > > the block is committed and clear all dirty bits from freed blocks. > > But for more careless filesystems, if they reallocate metadata block > > as a data block and don't clear the dirty bit in blockdev mapping, > > unmap_underlying_blocks() does it for them. > > That's what I thought - so I was wondering why we have specific calls to > this in ext4: > > mpage_da_map_blocks > __unmap_underlying_blocks > for (i = 0; i < blocks; i++) > unmap_underlying_metadata Hmm, OK. So maybe change it warn on dirty blockdev buffer and if the warning does not trigger we can believe that our theory is right ;). Honza -- Jan Kara SUSE Labs, CR