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Fri, 4 Dec 2020 02:05:53 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.21]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EBEC95C1CF; Fri, 4 Dec 2020 02:05:52 +0000 (UTC) Received: from lists01.pubmisc.prod.ext.phx2.redhat.com (lists01.pubmisc.prod.ext.phx2.redhat.com [10.5.19.33]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 869234EEEE; Fri, 4 Dec 2020 02:05:52 +0000 (UTC) Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) by lists01.pubmisc.prod.ext.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id 0B423rmD000477 for ; Thu, 3 Dec 2020 21:03:53 -0500 Received: by smtp.corp.redhat.com (Postfix) id 7FC3E1349A; 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 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 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 X-loop: dm-devel@redhat.com Cc: axboe@kernel.dk, martin.petersen@oracle.com, jdorminy@redhat.com, bjohnsto@redhat.com, linux-block@vger.kernel.org, dm-devel@redhat.com Subject: Re: [dm-devel] [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking X-BeenThere: dm-devel@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: device-mapper development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dm-devel-bounces@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 -- dm-devel mailing list dm-devel@redhat.com https://www.redhat.com/mailman/listinfo/dm-devel