From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from cuda.sgi.com (cuda1.sgi.com [192.48.157.11]) by oss.sgi.com (8.14.3/8.14.3/SuSE Linux 0.8) with ESMTP id o58JFZtU020712 for ; Tue, 8 Jun 2010 14:15:36 -0500 Received: from mail.parisc-linux.org (localhost [127.0.0.1]) by cuda.sgi.com (Spam Firewall) with ESMTP id EAF07118F797 for ; Thu, 29 Jul 2010 20:01:00 -0700 (PDT) Received: from mail.parisc-linux.org (palinux.external.hp.com [192.25.206.14]) by cuda.sgi.com with ESMTP id 148Pt7dVn7fE842b for ; Thu, 29 Jul 2010 20:01:00 -0700 (PDT) Date: Thu, 29 Jul 2010 20:53:24 -0600 From: Matthew Wilcox Subject: Re: [PATCH] dio: track and serialise unaligned direct IO Message-ID: <20100730025324.GO25774@parisc-linux.org> References: <1280443516-14448-1-git-send-email-david@fromorbit.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <1280443516-14448-1-git-send-email-david@fromorbit.com> List-Id: XFS Filesystem from SGI List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: xfs-bounces@oss.sgi.com Errors-To: xfs-bounces@oss.sgi.com To: Dave Chinner Cc: linux-fsdevel@vger.kernel.org, sandeen@sandeen.net, xfs@oss.sgi.com On Fri, Jul 30, 2010 at 08:45:16AM +1000, Dave Chinner wrote: > If we get two unaligned direct IO's to the same filesystem block > that is marked as a new allocation (i.e. buffer_new), then both IOs will > zero the portion of the block they are not writing data to. As a > result, when the IOs complete there will be a portion of the block > that contains zeros from the last IO to complete rather than the > data that should be there. Urgh. Yuck. > This is easily manifested by qemu using aio+dio with an unaligned > guest filesystem - every IO is unaligned and fileystem corruption is > encountered in the guest filesystem. xfstest 240 (from Eric Sandeen) > is also a simple reproducer. > > To avoid this problem, track unaligned IO that triggers sub-block zeroing and > check new incoming unaligned IO that require sub-block zeroing against that > list. If we get an overlap where the start and end of unaligned IOs hit the > same filesystem block, then we need to block the incoming IOs until the IO that > is zeroing the block completes. The blocked IO can then continue without > needing to do any zeroing and hence won't overwrite valid data with zeros. Urgh. Yuck. Could we perhaps handle this by making an IO instantiate a page cache page for partial writes, and forcing that portion of the IO through the page cache? The second IO would hit the same page and use the existing O_DIRECT vs page cache paths. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step." _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs