From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EBF5DC352A3 for ; Tue, 11 Feb 2020 20:42:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D41A620659 for ; Tue, 11 Feb 2020 20:42:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728655AbgBKUmY (ORCPT ); Tue, 11 Feb 2020 15:42:24 -0500 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:36152 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728512AbgBKUmY (ORCPT ); Tue, 11 Feb 2020 15:42:24 -0500 Received: from dread.disaster.area (pa49-179-138-28.pa.nsw.optusnet.com.au [49.179.138.28]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id C35BA7EAD5E; Wed, 12 Feb 2020 07:42:20 +1100 (AEDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1j1cMO-0002wz-8Z; Wed, 12 Feb 2020 07:42:20 +1100 Date: Wed, 12 Feb 2020 07:42:20 +1100 From: Dave Chinner To: Ira Weiny Cc: linux-kernel@vger.kernel.org, Alexander Viro , "Darrick J. Wong" , Dan Williams , Christoph Hellwig , "Theodore Y. Ts'o" , Jan Kara , linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v3 06/12] fs/xfs: Check if the inode supports DAX under lock Message-ID: <20200211204220.GN10776@dread.disaster.area> References: <20200208193445.27421-1-ira.weiny@intel.com> <20200208193445.27421-7-ira.weiny@intel.com> <20200211061639.GH10776@dread.disaster.area> <20200211175509.GD12866@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211175509.GD12866@iweiny-DESK2.sc.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=X6os11be c=1 sm=1 tr=0 a=zAxSp4fFY/GQY8/esVNjqw==:117 a=zAxSp4fFY/GQY8/esVNjqw==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=kj9zAlcOel0A:10 a=l697ptgUJYAA:10 a=QyXUC8HyAAAA:8 a=7-415B0cAAAA:8 a=dz0gKRpfQL60cEkPPx4A:9 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, Feb 11, 2020 at 09:55:09AM -0800, Ira Weiny wrote: > On Tue, Feb 11, 2020 at 05:16:39PM +1100, Dave Chinner wrote: > > On Sat, Feb 08, 2020 at 11:34:39AM -0800, ira.weiny@intel.com wrote: > > > From: Ira Weiny > > > > > > One of the checks for an inode supporting DAX is if the inode is > > > reflinked. During a non-DAX to DAX state change we could race with > > > the file being reflinked and end up with a reflinked file being in DAX > > > state. > > > > > > Prevent this race by checking for DAX support under the MMAP_LOCK. > > > > The on disk inode flags are protected by the XFS_ILOCK, not the > > MMAP_LOCK. i.e. the MMAPLOCK provides data access serialisation, not > > metadata modification serialisation. > > Ah... > > > > > > > > > Signed-off-by: Ira Weiny > > > --- > > > fs/xfs/xfs_ioctl.c | 11 +++++++---- > > > 1 file changed, 7 insertions(+), 4 deletions(-) > > > > > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c > > > index da1eb2bdb386..4ff402fd6636 100644 > > > --- a/fs/xfs/xfs_ioctl.c > > > +++ b/fs/xfs/xfs_ioctl.c > > > @@ -1194,10 +1194,6 @@ xfs_ioctl_setattr_dax_invalidate( > > > > > > *join_flags = 0; > > > > > > - if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX && > > > - !xfs_inode_supports_dax(ip)) > > > - return -EINVAL; > > > - > > > /* If the DAX state is not changing, we have nothing to do here. */ > > > if ((fa->fsx_xflags & FS_XFLAG_DAX) && > > > (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX)) > > > @@ -1211,6 +1207,13 @@ xfs_ioctl_setattr_dax_invalidate( > > > > > > /* lock, flush and invalidate mapping in preparation for flag change */ > > > xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL); > > > + > > > + if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX && > > > + !xfs_inode_supports_dax(ip)) { > > > + error = -EINVAL; > > > + goto out_unlock; > > > + } > > > > Yes, you might be able to get away with reflink vs dax flag > > serialisation on the inode flag modification, but it is not correct and > > leaves a landmine for future inode flag modifications that are done > > without holding either the MMAP or IOLOCK. > > > > e.g. concurrent calls to xfs_ioctl_setattr() setting/clearing flags > > other than the on disk DAX flag are all serialised by the ILOCK_EXCL > > and will no be serialised against this DAX check. Hence if there are > > other flags that we add in future that affect the result of > > xfs_inode_supports_dax(), this code will not be correctly > > serialised. > > > > This raciness in checking the DAX flags is the reason that > > xfs_ioctl_setattr_xflags() redoes all the reflink vs dax checks once > > it's called under the XFS_ILOCK_EXCL during the actual change > > transaction.... > > Ok I found this by trying to make sure that the xfs_inode_supports_dax() call > was always returning valid data. So I don't have a specific test which was > failing. > > Looking at the code again, it sounds like I was wrong about which locks protect > what and with your explanation above it looks like there is nothing to be done > here and I can drop the patch. > > Would you agree? *nod* Cheers, Dave. -- Dave Chinner david@fromorbit.com