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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 E2D58CA9EB9 for ; Mon, 21 Oct 2019 22:49:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C2C4A2084C for ; Mon, 21 Oct 2019 22:49:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730450AbfJUWtd (ORCPT ); Mon, 21 Oct 2019 18:49:33 -0400 Received: from mga07.intel.com ([134.134.136.100]:59581 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730276AbfJUWtd (ORCPT ); Mon, 21 Oct 2019 18:49:33 -0400 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga105.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 21 Oct 2019 15:49:32 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.67,325,1566889200"; d="scan'208";a="209483476" Received: from iweiny-desk2.sc.intel.com ([10.3.52.157]) by orsmga002.jf.intel.com with ESMTP; 21 Oct 2019 15:49:31 -0700 Date: Mon, 21 Oct 2019 15:49:31 -0700 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 5/5] fs/xfs: Allow toggle of physical DAX flag Message-ID: <20191021224931.GA25526@iweiny-DESK2.sc.intel.com> References: <20191020155935.12297-1-ira.weiny@intel.com> <20191020155935.12297-6-ira.weiny@intel.com> <20191021004536.GD8015@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191021004536.GD8015@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 Mon, Oct 21, 2019 at 11:45:36AM +1100, Dave Chinner wrote: > On Sun, Oct 20, 2019 at 08:59:35AM -0700, ira.weiny@intel.com wrote: > > @@ -1232,12 +1233,10 @@ xfs_diflags_to_linux( > > inode->i_flags |= S_NOATIME; > > else > > inode->i_flags &= ~S_NOATIME; > > -#if 0 /* disabled until the flag switching races are sorted out */ > > if (xflags & FS_XFLAG_DAX) > > inode->i_flags |= S_DAX; > > else > > inode->i_flags &= ~S_DAX; > > -#endif > > This code has bit-rotted. See xfs_setup_iops(), where we now have a > different inode->i_mapping->a_ops for DAX inodes. :-( > > That, fundamentally, is the issue here - it's not setting/clearing > the DAX flag that is the issue, it's doing a swap of the > mapping->a_ops while there may be other code using that ops > structure. > > IOWs, if there is any code anywhere in the kernel that > calls an address space op without holding one of the three locks we > hold here (i_rwsem, MMAPLOCK, ILOCK) then it can race with the swap > of the address space operations. > > By limiting the address space swap to file sizes of zero, we rule > out the page fault path (mmap of a zero length file segv's with an > access beyond EOF on the first read/write page fault, right?). Yes I checked that and thought we were safe here... > However, other aops callers that might run unlocked and do the wrong > thing if the aops pointer is swapped between check of the aop method > existing and actually calling it even if the file size is zero? > > A quick look shows that FIBMAP (ioctl_fibmap())) looks susceptible > to such a race condition with the current definitions of the XFS DAX > aops. I'm guessing there will be others, but I haven't looked > further than this... I'll check for others and think on what to do about this. ext4 will have the same problem I think. :-( I don't suppose using a single a_ops for both DAX and non-DAX is palatable? > > > /* lock, flush and invalidate mapping in preparation for flag change */ > > xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL); > > + > > + if (i_size_read(inode) != 0) { > > + error = -EOPNOTSUPP; > > + goto out_unlock; > > + } > > Wrong error. Should be the same as whatever is returned when we try > to change the extent size hint and can't because the file is > non-zero in length (-EINVAL, I think). Also needs a comment > explainging why this check exists, and probably better written as > i_size_read() > 0 .... Done, done, and done. Thank you, Ira > > Cheers, > > Dave. > -- > Dave Chinner > david@fromorbit.com