From: Eric Sandeen Subject: Re: [RFC PATCH] mark buffer_head mapping preallocate area as new during write_begin with delayed allocation Date: Tue, 28 Apr 2009 14:35:56 -0500 Message-ID: <49F75A9C.6000307@redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Theodore Tso , Mingming Cao , linux-ext4@vger.kernel.org To: "Aneesh Kumar K.V" Return-path: Received: from mx2.redhat.com ([66.187.237.31]:36760 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753960AbZD1TgD (ORCPT ); Tue, 28 Apr 2009 15:36:03 -0400 In-Reply-To: <20090428185749.GA3275@skywalker> 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? -Eric