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 39CFEC433E1 for ; Tue, 25 Aug 2020 15:13:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20A3C20786 for ; Tue, 25 Aug 2020 15:13:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726790AbgHYPNB (ORCPT ); Tue, 25 Aug 2020 11:13:01 -0400 Received: from mx2.suse.de ([195.135.220.15]:57726 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726627AbgHYPM7 (ORCPT ); Tue, 25 Aug 2020 11:12:59 -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 1697FAD0B; Tue, 25 Aug 2020 15:13:28 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 78ACA1E1316; Tue, 25 Aug 2020 17:12:56 +0200 (CEST) Date: Tue, 25 Aug 2020 17:12:56 +0200 From: Jan Kara To: "Theodore Y. Ts'o" Cc: Christoph Hellwig , Jan Kara , linux-fsdevel@vger.kernel.org, yebin , linux-block@vger.kernel.org, Jens Axboe , Mark Fasheh , Joel Becker , Joseph Qi Subject: Re: [PATCH RFC 2/2] block: Do not discard buffers under a mounted filesystem Message-ID: <20200825151256.GD32298@quack2.suse.cz> References: <20200825120554.13070-1-jack@suse.cz> <20200825120554.13070-3-jack@suse.cz> <20200825121616.GA10294@infradead.org> <20200825141020.GA668551@mit.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200825141020.GA668551@mit.edu> 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 10:10:20, Theodore Y. Ts'o wrote: > (Adding the OCFS2 maintainers, since my possibly insane idea proposed > below would definitely impact them!) > > On Tue, Aug 25, 2020 at 01:16:16PM +0100, 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. 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. > > I wonder if we should go a step further, and stop using struct > buffer_head altogether in jbd2 and ext4 (as well as ocfs2). What about the cache coherency issues I've pointed out in my reply to Christoph? > This would involve moving whatever structure elements from the > buffer_head struct into journal_head, and manage writeback and reads > requests directly in jbd2. This would allow us to get detailed write > errors back, which is currently not possible from the buffer_head > infrastructure. > > The downside is this would be a pretty massive change in terms of LOC, > since we use struct buffer_head in a *huge* number of places. If > we're careful, most of it could be handled by a Coccinelle script to > rename "struct buffer_head" to "struct journal_head". Fortunately, we > don't actually use that much of the fs/buffer_head functions in > fs/{ext4,ocfs2}/*.c. > > One potentially tricky bit is that ocfs2 hasn't been converted to > using iomap, so it's still using __blockdev_direct_IO. So it's data > blocks for DIO would still have to use struct buffer_head (which means > the Coccinelle script won't really work for fs/ocfs2, without a lot of > manual rework) --- or ocfs2 would have to switched to use iomap at > least for DIO support. > > What do folks think? Otherwise yes, this would be doable although pretty invasive as you mention. Honza -- Jan Kara SUSE Labs, CR