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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 D9F55C5DF60 for ; Fri, 8 Nov 2019 13:46:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B8F022084D for ; Fri, 8 Nov 2019 13:46:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728702AbfKHNqK (ORCPT ); Fri, 8 Nov 2019 08:46:10 -0500 Received: from mx2.suse.de ([195.135.220.15]:39232 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726845AbfKHNqJ (ORCPT ); Fri, 8 Nov 2019 08:46:09 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 9750DACEF; Fri, 8 Nov 2019 13:46:07 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id CD4111E3BE4; Fri, 8 Nov 2019 14:46:06 +0100 (CET) Date: Fri, 8 Nov 2019 14:46:06 +0100 From: Jan Kara To: Ira Weiny Cc: Dave Chinner , 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: <20191108134606.GL20863@quack2.suse.cz> References: <20191020155935.12297-1-ira.weiny@intel.com> <20191020155935.12297-6-ira.weiny@intel.com> <20191021004536.GD8015@dread.disaster.area> <20191021224931.GA25526@iweiny-DESK2.sc.intel.com> <20191108131238.GK20863@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191108131238.GK20863@quack2.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-ext4-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Fri 08-11-19 14:12:38, Jan Kara wrote: > On Mon 21-10-19 15:49:31, Ira Weiny wrote: > > 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: > > > 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. :-( > > Just as a datapoint, ext4 is bold and sets inode->i_mapping->a_ops on > existing inodes when switching journal data flag and so far it has not > blown up. What we did to deal with issues Dave describes is that we > introduced percpu rw-semaphore guarding switching of aops and then inside > problematic functions redirect callbacks in the right direction under this > semaphore. Somewhat ugly but it seems to work. Thinking about this some more, perhaps this scheme could be actually transformed in something workable. We could have a global (or maybe per-sb but I'm not sure it's worth it) percpu rwsem and we could transform aops calls into: percpu_down_read(aops_rwsem); do_call(); percpu_up_read(aops_rwsem); With some macro magic it needn't be even that ugly. Honza -- Jan Kara SUSE Labs, CR