From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: with ECARTIS (v1.0.0; list xfs); Wed, 24 Jan 2007 14:48:11 -0800 (PST) Received: from larry.melbourne.sgi.com (larry.melbourne.sgi.com [134.14.52.130]) by oss.sgi.com (8.12.10/8.12.10/SuSE Linux 0.7) with SMTP id l0OMm1qw003023 for ; Wed, 24 Jan 2007 14:48:03 -0800 Date: Thu, 25 Jan 2007 09:46:54 +1100 From: David Chinner Subject: Re: [PATCH 1/2]: Fix BUG in cancel_dirty_pages on XFS Message-ID: <20070124224654.GN33919298@melbourne.sgi.com> References: <20070123223702.GF33919298@melbourne.sgi.com> <1169640835.6189.14.camel@twins> <45B7627B.8050202@yahoo.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <45B7627B.8050202@yahoo.com.au> Sender: xfs-bounce@oss.sgi.com Errors-to: xfs-bounce@oss.sgi.com List-Id: xfs To: Nick Piggin Cc: Peter Zijlstra , David Chinner , linux-kernel@vger.kernel.org, xfs@oss.sgi.com, akpm@osdl.org On Thu, Jan 25, 2007 at 12:43:23AM +1100, Nick Piggin wrote: > Peter Zijlstra wrote: > >On Wed, 2007-01-24 at 09:37 +1100, David Chinner wrote: > > > >>With the recent changes to cancel_dirty_pages(), XFS will > >>dump warnings in the syslog because it can truncate_inode_pages() > >>on dirty mapped pages. > >> > >>I've determined that this is indeed correct behaviour for XFS > >>as this can happen in the case of races on mmap()d files with > >>direct I/O. In this case when we do a direct I/O read, we > >>flush the dirty pages to disk, then truncate them out of the > >>page cache. Unfortunately, between the flush and the truncate > >>the mmap could dirty the page again. At this point we toss a > >>dirty page that is mapped. > > > > > >This sounds iffy, why not just leave the page in the pagecache if its > >mapped anyway? > > And why not just leave it in the pagecache and be done with it? because what is in cache is then not coherent with what is on disk, and a direct read is supposed to read the data that is present in the file at the time it is issued. > All you need is to do a writeout before a direct IO read, which is > what generic dio code does. No, that's not good enough - after writeout but before the direct I/O read is issued a process can fault the page and dirty it. If you do a direct read, followed by a buffered read you should get the same data. The only way to guarantee this is to chuck out any cached pages across the range of the direct I/O so they are fetched again from disk on the next buffered I/O. i.e. coherent at the time the direct I/O is issued. > I guess you'll say that direct writes still need to remove pages, Yup. > but in that case you'll either have to live with some racyness > (which is what the generic code does), or have a higher level > synchronisation to prevent buffered + direct IO writes I suppose? The XFS inode iolock - direct I/O writes take it shared, buffered writes takes it exclusive - so you can't do both at once. Buffered reads take is shared, which is another reason why we need to purge the cache on direct I/O writes - they can operate concurrently (and coherently) with buffered reads. Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group