From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.fusionio.com ([66.114.96.30]:58000 "EHLO mx1.fusionio.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754038Ab2F2NFN (ORCPT ); Fri, 29 Jun 2012 09:05:13 -0400 Date: Fri, 29 Jun 2012 09:05:10 -0400 From: Chris Mason To: Miao Xie CC: Josef Bacik , "linux-btrfs@vger.kernel.org" Subject: Re: [PATCH] Btrfs: fix dio write vs buffered read race V2 Message-ID: <20120629130510.GY17638@shiny> References: <1340718176-4999-1-git-send-email-jbacik@fusionio.com> <4FEBD0EC.6070802@cn.fujitsu.com> <20120628123422.GB1729@localhost.localdomain> <4FED107B.9020308@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: <4FED107B.9020308@cn.fujitsu.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Thu, Jun 28, 2012 at 08:18:35PM -0600, Miao Xie wrote: > On Thu, 28 Jun 2012 08:34:23 -0400, Josef Bacik wrote: > > On Wed, Jun 27, 2012 at 09:35:08PM -0600, Miao Xie wrote: > >> On Tue, 26 Jun 2012 09:42:56 -0400, Josef Bacik wrote: > >>> From: Josef Bacik > >>> > >>> Miao pointed out there's a problem with mixing dio writes and buffered > >>> reads. If the read happens between us invalidating the page range and > >>> actually locking the extent we can bring in pages into page cache. Then > >>> once the write finishes if somebody tries to read again it will just find > >>> uptodate pages and we'll read stale data. So we need to lock the extent and > >>> check for uptodate bits in the range. If there are uptodate bits we need to > >>> unlock and invalidate again. This will keep this race from happening since > >>> we will hold the extent locked until we create the ordered extent, and then > >>> teh read side always waits for ordered extents. Thanks, > >> > >> This patch still can not work well. It is because we don't update i_size in time. > >> Writer Worker Reader > >> lock_extent > >> do direct io > >> end io > >> finish io > >> unlock_extent > >> lock_extent > >> check the pos is beyond EOF or not > >> beyond EOF, zero the page and set it uptodate > >> unlock_extent > >> update i_size > >> > >> So I think we must update the i_size in time, and I wrote a small patch to do it: > >> > > > > We should probably be updating i_size when we create an extent past EOF in the > > write stuff, not during endio, I will work this out and fold it into my patch. > > Good catch. > > It is better that update i_size in endio, I think. because during endio, we are sure that > the data is flushed into the disk successfully, and can update i_size at ease. and if the > error happens when flushing the data into the disk, we also needn't reset i_size. I think the i_size update should happen sooner. The rest of the filesystems work that way, and it will have fewer interaction problems with the VM. -chris