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=-13.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 BB7F1C1B0D9 for ; Fri, 4 Dec 2020 16:49:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8F0B3229C9 for ; Fri, 4 Dec 2020 16:49:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388205AbgLDQtt (ORCPT ); Fri, 4 Dec 2020 11:49:49 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:51828 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387952AbgLDQtt (ORCPT ); Fri, 4 Dec 2020 11:49:49 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607100501; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=73m+KJ0HRIRsozVJo8fE+/vehD0NPYPNmHifBwELEI4=; b=RtITRYYQSe9QUiIK4Mk6zgOh7o59otMf1bgvoB2VAJ2cwqM2e2LWkNz5yAbrJadyIpT5CY /XH14oWQ07UAnwaR3z95c+8EYbwuFuRNSAZqYeFFc80uh9Lc/BijejrW11p9HPa6/82pc7 eisv2A8uqDrSLKq26gOQJuQzbmoTtc4= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-560-0gbKLN_9O2-MNqIU-Yj70Q-1; Fri, 04 Dec 2020 11:48:19 -0500 X-MC-Unique: 0gbKLN_9O2-MNqIU-Yj70Q-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B6F46107ACE6; Fri, 4 Dec 2020 16:48:18 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 45E125D9DC; Fri, 4 Dec 2020 16:47:59 +0000 (UTC) Date: Fri, 4 Dec 2020 11:47:59 -0500 From: Mike Snitzer To: Ming Lei Cc: axboe@kernel.dk, martin.petersen@oracle.com, linux-block@vger.kernel.org, dm-devel@redhat.com, jdorminy@redhat.com, bjohnsto@redhat.com Subject: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking Message-ID: <20201204164759.GA2761@redhat.com> References: <20201130171805.77712-1-snitzer@redhat.com> <20201201160709.31748-1-snitzer@redhat.com> <20201203032608.GD540033@T590> <20201203143359.GA29261@redhat.com> <20201204011243.GB661914@T590> <20201204020343.GA32150@redhat.com> <20201204035924.GD661914@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201204035924.GD661914@T590> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Dec 03 2020 at 10:59pm -0500, Ming Lei wrote: > On Thu, Dec 03, 2020 at 09:03:43PM -0500, Mike Snitzer wrote: > > On Thu, Dec 03 2020 at 8:12pm -0500, > > Ming Lei wrote: > > > > > On Thu, Dec 03, 2020 at 09:33:59AM -0500, Mike Snitzer wrote: > > > > On Wed, Dec 02 2020 at 10:26pm -0500, > > > > Ming Lei wrote: > > > > > > > > > On Tue, Dec 01, 2020 at 11:07:09AM -0500, Mike Snitzer wrote: > > > > > > commit 22ada802ede8 ("block: use lcm_not_zero() when stacking > > > > > > chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must > > > > > > reflect the most limited of all devices in the IO stack. > > > > > > > > > > > > Otherwise malformed IO may result. E.g.: prior to this fix, > > > > > > ->chunk_sectors = lcm_not_zero(8, 128) would result in > > > > > > blk_max_size_offset() splitting IO at 128 sectors rather than the > > > > > > required more restrictive 8 sectors. > > > > > > > > > > What is the user-visible result of splitting IO at 128 sectors? > > > > > > > > The VDO dm target fails because it requires IO it receives to be split > > > > as it advertised (8 sectors). > > > > > > OK, looks VDO's chunk_sector limit is one hard constraint, even though it > > > is one DM device, so I guess you are talking about DM over VDO? > > > > > > Another reason should be that VDO doesn't use blk_queue_split(), otherwise it > > > won't be a trouble, right? > > > > > > Frankly speaking, if the stacking driver/device has its own hard queue limit > > > like normal hardware drive, the driver should be responsible for the splitting. > > > > DM core does the splitting for VDO (just like any other DM target). > > In 5.9 I updated DM to use chunk_sectors, use blk_stack_limits() > > stacking of it, and also use blk_max_size_offset(). > > > > But all that block core code has shown itself to be too rigid for DM. I > > tried to force the issue by stacking DM targets' ti->max_io_len with > > chunk_sectors. But really I'd need to be able to pass in the per-target > > max_io_len to blk_max_size_offset() to salvage using it. > > > > Stacking chunk_sectors seems ill-conceived. One size-fits-all splitting > > is too rigid. > > DM/VDO knows exactly it is one hard chunk_sectors limit, and DM shouldn't play > the stacking trick on VDO's chunk_sectors limit, should it? Feel like I already answered this in detail but... correct, DM cannot and should not use stacked chunk_sectors as basis for splitting. Up until 5.9, where I changed DM core to set and then use chunk_sectors for splitting via blk_max_size_offset(), DM only used its own per-target ti->max_io_len in drivers/md/dm.c:max_io_len(). But I reverted back to DM's pre-5.9 splitting in this stable@ fix that I'll be sending to Linus today for 5.10-rcX: https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10-rcX&id=6bb38bcc33bf3093c08bd1b71e4f20c82bb60dd1 DM is now back to pre-5.9 behavior where it doesn't even consider chunk_sectors for splitting (NOTE: dm-zoned sets ti->max_io_len though so it is effectively achieves the same boundary splits via max_io_len). With that baseline established, what I'm now saying is: if DM, the most common limits stacking consumer, cannot benefit from stacked chunk_sectors then what stacked device does benefit? Could be block core's stacked chunk_sectors based splitting is good enough for others, just not yet seeing how. Feels like it predates blk_queue_split() and the stacking of chunk_sectors could/should be removed now. All said, I'm fine with leaving stacked chunk_sectors for others to care about... think I've raised enough awareness on this topic now ;) Mike