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 44DEBC3B189 for ; Tue, 11 Feb 2020 17:55:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3033B20661 for ; Tue, 11 Feb 2020 17:55:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730727AbgBKRzK (ORCPT ); Tue, 11 Feb 2020 12:55:10 -0500 Received: from mga12.intel.com ([192.55.52.136]:27492 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729205AbgBKRzK (ORCPT ); Tue, 11 Feb 2020 12:55:10 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 11 Feb 2020 09:55:09 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,428,1574150400"; d="scan'208";a="266342730" Received: from iweiny-desk2.sc.intel.com ([10.3.52.157]) by fmsmga002.fm.intel.com with ESMTP; 11 Feb 2020 09:55:09 -0800 Date: Tue, 11 Feb 2020 09:55:09 -0800 From: Ira Weiny To: Dave Chinner 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: <20200211175509.GD12866@iweiny-DESK2.sc.intel.com> References: <20200208193445.27421-1-ira.weiny@intel.com> <20200208193445.27421-7-ira.weiny@intel.com> <20200211061639.GH10776@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200211061639.GH10776@dread.disaster.area> User-Agent: Mutt/1.11.1 (2018-12-01) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org 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? Thanks for the review! Ira > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com