From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758523AbXFGHVI (ORCPT ); Thu, 7 Jun 2007 03:21:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751209AbXFGHU5 (ORCPT ); Thu, 7 Jun 2007 03:20:57 -0400 Received: from brick.kernel.dk ([80.160.20.94]:1278 "EHLO kernel.dk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752153AbXFGHU4 (ORCPT ); Thu, 7 Jun 2007 03:20:56 -0400 Date: Thu, 7 Jun 2007 09:19:30 +0200 From: Jens Axboe To: NeilBrown Cc: Andrew Morton , linux-kernel@vger.kernel.org, Nick Piggin Subject: Re: [PATCH 001 of 2] Fix read/truncate race. Message-ID: <20070607071929.GL4735@kernel.dk> References: <20070607114043.26967.patches@notabene> <1070607014653.27304@suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1070607014653.27304@suse.de> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 07 2007, NeilBrown wrote: > > do_generic_mapping_read currently samples the i_size at the start > and doesn't do so again unless it needs to call ->readpage to load > a page. After ->readpage it has to re-sample i_size as a truncate > may have caused that page to be filled with zeros, and the read() > call should not see these. > > However there are other activities that might cause ->readpage to be > called on a page between the time that do_generic_mapping_read > samples i_size and when it finds that it has an uptodate page. These > include at least read-ahead and possibly another thread performing a > read. > > So do_generic_mapping_read must sample i_size *after* it has an > uptodate page. Thus the current sampling at the start and after a read > can be replaced with a sampling before the copy-out. > > The same change applied to __generic_file_splice_read. > > Note that this fixes any race with truncate_complete_page, but does > not fix a possible race with truncate_partial_page. If a partial > truncate happens after do_generic_mapping_read samples i_size and > before the copy_out, the nuls that truncate_partial_page place in the > page could be copied out incorrectly. > > I think the best fix for that is to *not* zero out parts of the page > in truncate_partial_page, but rather to zero out the tail of a page > when increasing i_size. Thanks Neil, if you don't mind I'll steal the splice bits and add them to the splice branch, as this particular patch conflicts with other isize fixes that Hugh and I have been working on. -- Jens Axboe