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=-5.5 required=3.0 tests=BAYES_00, 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 340E4C433E1 for ; Tue, 28 Jul 2020 02:21:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 08E8020672 for ; Tue, 28 Jul 2020 02:21:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726313AbgG1CVG (ORCPT ); Mon, 27 Jul 2020 22:21:06 -0400 Received: from mail105.syd.optusnet.com.au ([211.29.132.249]:55459 "EHLO mail105.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726269AbgG1CVG (ORCPT ); Mon, 27 Jul 2020 22:21:06 -0400 Received: from dread.disaster.area (pa49-180-53-24.pa.nsw.optusnet.com.au [49.180.53.24]) by mail105.syd.optusnet.com.au (Postfix) with ESMTPS id 84650372EE2; Tue, 28 Jul 2020 12:21:01 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1k0FEh-0002IA-R0; Tue, 28 Jul 2020 12:20:59 +1000 Date: Tue, 28 Jul 2020 12:20:59 +1000 From: Dave Chinner To: "Li, Hao" Cc: "linux-xfs@vger.kernel.org" , "linux-nvdimm@lists.01.org" , "ira.weiny@intel.com" , "Gotou, Yasunori" Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches? Message-ID: <20200728022059.GX2005@dread.disaster.area> References: <9dc179147f6a47279d801445f3efeecc@G08CNEXMBPEKD04.g08.fujitsu.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9dc179147f6a47279d801445f3efeecc@G08CNEXMBPEKD04.g08.fujitsu.local> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=QIgWuTDL c=1 sm=1 tr=0 a=moVtWZxmCkf3aAMJKIb/8g==:117 a=moVtWZxmCkf3aAMJKIb/8g==:17 a=kj9zAlcOel0A:10 a=_RQrkK6FrEwA:10 a=7-415B0cAAAA:8 a=6gdJAn9UP7HguOJFIc0A: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, Jul 28, 2020 at 02:00:08AM +0000, Li, Hao wrote: > Hi, > > I have noticed that we have to drop caches to make the changing of S_DAX > flag take effect after using chattr +x to turn on DAX for a existing > regular file. The related function is xfs_diflags_to_iflags, whose > second parameter determines whether we should set S_DAX immediately. Yup, as documented in Documentation/filesystems/dax.txt. Specifically: 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag, the change in behaviour for existing regular files may not occur immediately. If the change must take effect immediately, the administrator needs to: a) stop the application so there are no active references to the data set the policy change will affect b) evict the data set from kernel caches so it will be re-instantiated when the application is restarted. This can be achieved by: i. drop-caches ii. a filesystem unmount and mount cycle iii. a system reboot > I can't figure out why we do this. Is this because the page caches in > address_space->i_pages are hard to deal with? Because of unfixable races in the page fault path that prevent changing the caching behaviour of the inode while concurrent access is possible. The only way to guarantee races can't happen is to cycle the inode out of cache. > I also wonder what will > happen if we set S_DAX unconditionally. Thanks! As per the documentation, that's exactly what the dax=always mount option does. Cheers, Dave. -- Dave Chinner david@fromorbit.com