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=-6.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 5EEF8C433E0 for ; Fri, 31 Jul 2020 09:12:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4571821883 for ; Fri, 31 Jul 2020 09:12:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732044AbgGaJMq (ORCPT ); Fri, 31 Jul 2020 05:12:46 -0400 Received: from mail.cn.fujitsu.com ([183.91.158.132]:42576 "EHLO heian.cn.fujitsu.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731991AbgGaJMq (ORCPT ); Fri, 31 Jul 2020 05:12:46 -0400 X-IronPort-AV: E=Sophos;i="5.75,417,1589212800"; d="scan'208";a="97212846" Received: from unknown (HELO cn.fujitsu.com) ([10.167.33.5]) by heian.cn.fujitsu.com with ESMTP; 31 Jul 2020 17:12:42 +0800 Received: from G08CNEXMBPEKD04.g08.fujitsu.local (unknown [10.167.33.201]) by cn.fujitsu.com (Postfix) with ESMTP id 0E4604CE4B09; Fri, 31 Jul 2020 17:12:39 +0800 (CST) Received: from [10.167.225.206] (10.167.225.206) by G08CNEXMBPEKD04.g08.fujitsu.local (10.167.33.201) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Fri, 31 Jul 2020 17:12:40 +0800 Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches? To: Ira Weiny CC: Dave Chinner , "linux-xfs@vger.kernel.org" , "linux-nvdimm@lists.01.org" , , , , Yasunori Goto References: <9dc179147f6a47279d801445f3efeecc@G08CNEXMBPEKD04.g08.fujitsu.local> <20200728022059.GX2005@dread.disaster.area> <573feb69-bc38-8eb4-ee9b-7c49802eb737@fujitsu.com> <20200729161040.GA1250504@iweiny-DESK2.sc.intel.com> From: "Li, Hao" Message-ID: <5717e1e5-79fb-af3c-0859-eea3cd8d9626@cn.fujitsu.com> Date: Fri, 31 Jul 2020 17:12:38 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.0.1 MIME-Version: 1.0 In-Reply-To: <20200729161040.GA1250504@iweiny-DESK2.sc.intel.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Language: en-US X-Originating-IP: [10.167.225.206] X-ClientProxiedBy: G08CNEXCHPEKD06.g08.fujitsu.local (10.167.33.205) To G08CNEXMBPEKD04.g08.fujitsu.local (10.167.33.201) X-yoursite-MailScanner-ID: 0E4604CE4B09.AF3EC X-yoursite-MailScanner: Found to be clean X-yoursite-MailScanner-From: lihao2018.fnst@cn.fujitsu.com Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On 2020/7/30 0:10, Ira Weiny wrote: > On Wed, Jul 29, 2020 at 11:23:21AM +0900, Yasunori Goto wrote: >> Hi, >> >> On 2020/07/28 11:20, Dave Chinner wrote: >>> 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 understand why the drop_cache operation is necessary. Thanks. >> >> BTW, even normal user becomes to able to change DAX flag for an inode, >> drop_cache operation still requires root permission, right? >> >> So, if kernel have a feature for normal user can operate drop cache for "a >> inode" with >> its permission, I think it improve the above limitation, and >> we would like to try to implement it recently. >> >> Do you have any opinion making such feature? >> (Agree/opposition, or any other comment?) > I would not be opposed but there were many hurdles to that implementation. > > What is the use case you are thinking of here? > > The compromise of dropping caches was reached because we envisioned that many > users would simply want to chose the file mode when a file was created and > maintain that mode through the lifetime of the file. To that end one can > simply create directories which have the desired dax mode and any files created > in that directory will inherit the dax mode immediately. Inheriting mechanism for DAX mode is reasonable but chattr&drop_caches makes things complicated. > So there is no need > to switch the file mode directly as a normal user. The question is, the normal users can indeed use chattr to change the DAX mode for a regular file as long as they want. However, when they do this, they have no way to make the change take effect. I think this behavior is weird. We can say chattr executes successfully because XFS_DIFLAG2_DAX has been set onto xfs_inode->i_d.di_flags2, but we can also say chattr doesn't finish things completely because S_DAX is not set onto inode->i_flags. The user may be confused about why chattr +/-x doesn't work at all. Maybe we should find a way for the normal user to make chattr take effects without calling the administrator, or we can make the chattr +/x command request root permission now that if the user has root permission, he can make DAX changing take effect through echo 2 > /proc/sys/vm/drop_caches. Regards, Hao Li > > Would that work for your use case? > > Ira