From: Theodore Tso Subject: Re: [RFC PATCH] mark buffer_head mapping preallocate area as new during write_begin with delayed allocation Date: Tue, 28 Apr 2009 13:00:47 -0400 Message-ID: <20090428170047.GC24043@mit.edu> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mingming Cao , sandeen@redhat.com, linux-ext4@vger.kernel.org To: "Aneesh Kumar K.V" Return-path: Received: from THUNK.ORG ([69.25.196.29]:36060 "EHLO thunker.thunk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760328AbZD1RA6 (ORCPT ); Tue, 28 Apr 2009 13:00:58 -0400 Content-Disposition: inline In-Reply-To: <20090428163554.GA27670@skywalker> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, Apr 28, 2009 at 10:05:54PM +0530, Aneesh Kumar K.V wrote: > On Tue, Apr 28, 2009 at 08:48:21AM -0400, Theodore Tso wrote: > > On Tue, Apr 28, 2009 at 03:01:45PM +0530, Aneesh Kumar K.V wrote: > > > > > > Looking at the source again i guess setting just b_dev is not enough. > > > unmap_underlying_metadata looks at the mapping block number, which we > > > don't have in case on unwritten buffer_head. How about the below patch ? > > > It involve vfs changes. But i guess it is correct with respect to the > > > meaning of BH_New (Disk mapping was newly created by get_block). I guess > > > BH_New implies BH_Mapped. > > > > Argh. So we have multiple problems going on here. One is the > > original problem, namely that of a partial write into an preallocated > > block can leave garbage behind in that unitialized block. > > > > 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? > The original reported problem is something really easy to reproduce. So > i guess if we can have a ext4 local change that would fix the original > problem that would be good. Considering that map_bh(bdev, 0) didn't > create any issues till now, what we can do is to do a similar update > for unwritten_buffer in ext4_da_block_write_prep. That's the v2 version > of the patch with the below addition > bh_result->b_blocknr = 0; OK, I can put togehter a patch to do this. Whatever we do, I think we're going to need a *lot* of testing. - Ted