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=-5.3 required=3.0 tests=BAYES_00, 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 8004EC433E3 for ; Tue, 25 Aug 2020 14:51:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5437A2075F for ; Tue, 25 Aug 2020 14:51:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726551AbgHYOvB (ORCPT ); Tue, 25 Aug 2020 10:51:01 -0400 Received: from mx2.suse.de ([195.135.220.15]:46774 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725998AbgHYOu7 (ORCPT ); Tue, 25 Aug 2020 10:50:59 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 1A251AB7D; Tue, 25 Aug 2020 14:51:27 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 1C4E61E1316; Tue, 25 Aug 2020 16:50:56 +0200 (CEST) Date: Tue, 25 Aug 2020 16:50:56 +0200 From: Jan Kara To: Christoph Hellwig Cc: 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: <20200825145056.GC32298@quack2.suse.cz> References: <20200825120554.13070-1-jack@suse.cz> <20200825120554.13070-3-jack@suse.cz> <20200825121616.GA10294@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200825121616.GA10294@infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue 25-08-20 13:16:16, Christoph Hellwig wrote: > On Tue, Aug 25, 2020 at 02:05:54PM +0200, Jan Kara wrote: > > Discarding blocks and buffers under a mounted filesystem is hardly > > anything admin wants to do. Usually it will confuse the filesystem and > > sometimes the loss of buffer_head state (including b_private field) can > > even cause crashes like: > > Doesn't work if the file system uses multiple devices. Hum, right. > I think we just really need to split the fs buffer_head address space > from the block device one. Everything else is just going to cause a huge > mess. 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? 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). So we would need to somehow maintain at least vague coherence between these caches which would be ugly. Honza -- Jan Kara SUSE Labs, CR