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 13:57:27 +0200 Message-ID: <20090429115727.GC18195@atrey.karlin.mff.cuni.cz> References: <1240859143-31122-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Aneesh Kumar K.V" , Theodore Tso , Mingming Cao , linux-ext4@vger.kernel.org To: Eric Sandeen Return-path: Received: from atrey.karlin.mff.cuni.cz ([195.113.26.193]:34282 "EHLO atrey.karlin.mff.cuni.cz" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754966AbZD2L52 (ORCPT ); Wed, 29 Apr 2009 07:57:28 -0400 Content-Disposition: inline In-Reply-To: <49F75A9C.6000307@redhat.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: > 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. Honza -- Jan Kara SuSE CR Labs