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 DB399C433DF for ; Wed, 29 Jul 2020 23:21:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C0FDF2067D for ; Wed, 29 Jul 2020 23:21:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727083AbgG2XVj (ORCPT ); Wed, 29 Jul 2020 19:21:39 -0400 Received: from mail109.syd.optusnet.com.au ([211.29.132.80]:43229 "EHLO mail109.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726876AbgG2XVj (ORCPT ); Wed, 29 Jul 2020 19:21:39 -0400 Received: from dread.disaster.area (pa49-180-53-24.pa.nsw.optusnet.com.au [49.180.53.24]) by mail109.syd.optusnet.com.au (Postfix) with ESMTPS id 8AA5AD7F054; Thu, 30 Jul 2020 09:21:35 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1k0vO7-0001KV-2E; Thu, 30 Jul 2020 09:21:31 +1000 Date: Thu, 30 Jul 2020 09:21:31 +1000 From: Dave Chinner To: Yasunori Goto Cc: "Li, Hao" , "linux-xfs@vger.kernel.org" , "linux-nvdimm@lists.01.org" , "ira.weiny@intel.com" Subject: Re: Can we change the S_DAX flag immediately on XFS without dropping caches? Message-ID: <20200729232131.GC2005@dread.disaster.area> References: <9dc179147f6a47279d801445f3efeecc@G08CNEXMBPEKD04.g08.fujitsu.local> <20200728022059.GX2005@dread.disaster.area> <573feb69-bc38-8eb4-ee9b-7c49802eb737@fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <573feb69-bc38-8eb4-ee9b-7c49802eb737@fujitsu.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=LPwYv6e9 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=_fcLz_GkVchqeGo6EiQA: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 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? Step back for a minute and explain why you want to be able to change the DAX mode of a file -as a user-. > 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. No, drop_caches is not going to be made available to users. That makes it s trivial system wide DoS vector. Cheers, Dave. -- Dave Chinner david@fromorbit.com