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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 6FB29C433E6 for ; Fri, 28 Aug 2020 00:07:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4E5F3208C9 for ; Fri, 28 Aug 2020 00:07:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726803AbgH1AHL (ORCPT ); Thu, 27 Aug 2020 20:07:11 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:36441 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726147AbgH1AHL (ORCPT ); Thu, 27 Aug 2020 20:07:11 -0400 Received: from dread.disaster.area (pa49-181-146-199.pa.nsw.optusnet.com.au [49.181.146.199]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id 8F40F824181; Fri, 28 Aug 2020 10:07:06 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1kBRv7-0007aY-7s; Fri, 28 Aug 2020 10:07:05 +1000 Date: Fri, 28 Aug 2020 10:07:05 +1000 From: Dave Chinner To: Al Viro Cc: Christoph Hellwig , Jan Kara , linux-fsdevel@vger.kernel.org, yebin , linux-block@vger.kernel.org, Jens Axboe Subject: Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem Message-ID: <20200828000705.GC12096@dread.disaster.area> References: <20200825120554.13070-1-jack@suse.cz> <20200825120554.13070-3-jack@suse.cz> <20200825121616.GA10294@infradead.org> <20200825145056.GC32298@quack2.suse.cz> <20200827071603.GA25305@infradead.org> <20200827213900.GG1236603@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200827213900.GG1236603@ZenIV.linux.org.uk> X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=XJ9OtjpE c=1 sm=1 tr=0 cx=a_idp_d a=GorAHYkI+xOargNMzM6qxQ==:117 a=GorAHYkI+xOargNMzM6qxQ==:17 a=kj9zAlcOel0A:10 a=y4yBn9ojGxQA:10 a=7-415B0cAAAA:8 a=Zuxhg74iZb8wFQBIpPYA:9 a=9wW33gk-GpalGAZl:21 a=1o6rFZfVsUrZ5jbQ:21 a=CjuIK1q_8ugA:10 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Aug 27, 2020 at 10:39:00PM +0100, Al Viro wrote: > On Thu, Aug 27, 2020 at 08:16:03AM +0100, Christoph Hellwig wrote: > > On Tue, Aug 25, 2020 at 04:50:56PM +0200, Jan Kara wrote: > > > Do you mean that address_space filesystem uses to access its metadata on > > > /dev/sda will be different from the address_space you will see when reading > > > say /dev/sda? Thus these will be completely separate (and incoherent) > > > caches? > > > > Yes. > > > > > Although this would be simple it will break userspace I'm afraid. > > > There are situations where tools read e.g. superblock of a mounted > > > filesystem from the block device and rely on the data to be reasonably > > > recent. Even worse e.g. tune2fs or e2fsck can *modify* superblock of a > > > mounted filesystem through the block device (e.g. to set 'fsck after X > > > mounts' fields and similar). > > > > We've not had any problems when XFS stopped using the block device > > address space 9.5 years ago. > > How much writes from fsck use does xfs see, again? All of them, because xfs_repair uses direct IO and caches what it needs in userspace. Cheers, Dave. -- Dave Chinner david@fromorbit.com