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=-8.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 786B9C433FE for ; Fri, 4 Dec 2020 02:05:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1DAC222511 for ; Fri, 4 Dec 2020 02:05:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726041AbgLDCFX (ORCPT ); Thu, 3 Dec 2020 21:05:23 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:37535 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725885AbgLDCFX (ORCPT ); Thu, 3 Dec 2020 21:05:23 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607047436; 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=qAzlATevKVsfS0B0Y6sohYhm6zY8rB2tZXdOJa1bHW4=; b=UUAvaFwBD58X5cX0lp8mOh+oFIgG9cIXDgH41e9enAJFhMJbUdLuW1PnIj5HZCwQOr+2Zi 4De8NzGIfhrMvMRLS31RmjXPZm1FgjDAS3CtvfGOMXHIcvXQSlPpWXv7sEl8MsdP7yv599 yaFPTg6znd/6ru9Yp24M7f7znGzTrQo= 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-128-tObFgy7AOBWF6v7hg4Sacw-1; Thu, 03 Dec 2020 21:03:54 -0500 X-MC-Unique: tObFgy7AOBWF6v7hg4Sacw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 80AED180A089; Fri, 4 Dec 2020 02:03:53 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 305FE620D7; Fri, 4 Dec 2020 02:03:44 +0000 (UTC) Date: Thu, 3 Dec 2020 21:03:43 -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: <20201204020343.GA32150@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201204011243.GB661914@T590> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. > > > I understand it isn't related with correctness, because the underlying > > > queue can split by its own chunk_sectors limit further. So is the issue > > > too many further-splitting on queue with chunk_sectors 8? then CPU > > > utilization is increased? Or other issue? > > > > No, this is all about correctness. > > > > Seems you're confining the definition of the possible stacking so that > > the top-level device isn't allowed to have its own hard requirements on > > I just don't know this story, thanks for your clarification. > > As I mentioned above, if the stacking driver has its own hard queue > limit, it should be the driver's responsibility to respect it via > blk_queue_split() or whatever. Again, DM does its own splitting... that aspect of it isn't an issue. The problem is the basis for splitting cannot be the stacked up chunk_sectors. Mike