Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752632AbbKYTKb (ORCPT ); Wed, 25 Nov 2015 14:10:31 -0500 Received: from mx2.suse.de ([195.135.220.15]:39656 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752120AbbKYTKW (ORCPT ); Wed, 25 Nov 2015 14:10:22 -0500 Subject: Re: kernel BUG at drivers/scsi/scsi_lib.c:1096! To: Jens Axboe , emilne@redhat.com References: <1447838334.1564.2.camel@ellerman.id.au> <1447855399.3974.24.camel@redhat.com> <1447894964.15206.0.camel@ellerman.id.au> <20151119082325.GA11419@infradead.org> <564DEC41.5010600@suse.de> <1448030316.4067.18.camel@localhost.localdomain> <564F3453.9040603@suse.de> <1448033323.4067.24.camel@localhost.localdomain> <565579A2.4000005@suse.de> <5655F635.5040805@fb.com> Cc: Christoph Hellwig , Michael Ellerman , Mark Salter , "James E. J. Bottomley" , brking , linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-block@vger.kernel.org, Mike Snitzer , "Jun'ichi Nomura" From: Hannes Reinecke Message-ID: <5656079A.3040805@suse.de> Date: Wed, 25 Nov 2015 20:10:18 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <5655F635.5040805@fb.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3610 Lines: 98 On 11/25/2015 06:56 PM, Jens Axboe wrote: > On 11/25/2015 02:04 AM, Hannes Reinecke wrote: >> On 11/20/2015 04:28 PM, Ewan Milne wrote: >>> On Fri, 2015-11-20 at 15:55 +0100, Hannes Reinecke wrote: >>>> Can't we have a joint effort here? >>>> I've been spending a _LOT_ of time trying to debug things here, but >>>> none of the ideas I've come up with have been able to fix anything. >>> >>> Yes. I'm not the one primarily looking at it, and we don't have a >>> reproducer in-house. We just have the one dump right now. >>> >>>> >>>> I'm almost tempted to increase the count from scsi_alloc_sgtable() >>>> by one and be done with ... >>>> >>> >>> That might not fix it if it is a problem with the merge code, though. >>> >> And indeed, it doesn't. >> Seems I finally found the culprit. >> >> What happens is this: >> We have two paths, with these seg_boundary_masks: >> >> path-1: seg_boundary_mask = 65535, >> path-2: seg_boundary_mask = 4294967295, >> >> consequently the DM request queue has this: >> >> md-1: seg_boundary_mask = 65535, >> >> What happens now is that a request is being formatted, and sent >> to path 2. During submission req->nr_phys_segments is formatted >> with the limits of path 2, arriving at a count of 3. >> Now the request gets retried on path 1, but as the NOMERGE request >> flag is set req->nr_phys_segments is never updated. >> But blk_rq_map_sg() ignores all counters, and just uses the >> bi_vec directly, resulting in a count of 4 -> boom. >> >> So the culprit here is the NOMERGE flag, which is evaluated >> via >> ->dm_dispatch_request() >> ->blk_insert_cloned_request() >> ->blk_rq_check_limits() >> >> If the above assessment is correct, the following patch should >> fix it: >> >> diff --git a/block/blk-core.c b/block/blk-core.c >> index 801ced7..12cccd6 100644 >> --- a/block/blk-core.c >> +++ b/block/blk-core.c >> @@ -1928,7 +1928,7 @@ EXPORT_SYMBOL(submit_bio); >> */ >> int blk_rq_check_limits(struct request_queue *q, struct request *rq) >> { >> - if (!rq_mergeable(rq)) >> + if (rq->cmd_type != REQ_TYPE_FS) >> return 0; >> >> if (blk_rq_sectors(rq) > blk_queue_get_max_sectors(q, >> rq->cmd_flags)) { >> >> >> Mike? Jens? >> Can you comment on it? > > We only support merging on REQ_TYPE_FS already, so how is the above > making it any different? In general, NOMERGE being set or not should not > make a difference. It's only a hint that we need not check further if we > should be merging on this request, since we already tried it once, found > we'd exceed various limits, then set NOMERGE to reflect that. > The problem is that NOMERGE does too much, as it inhibits _any_ merging. Unfortunately, the req->nr_phys_segments value is evaluated in the final _driver_ context _after_ the merging happend; cf scsi_lib.c:scsi_init_sgtable(). As nr_phys_segments is inherited from the original request (and never recalculated with the new request queue limits) the following blk_rq_map_sg() call might end up at a different calculation, especially after retrying a request on another path. Cheers, Hannes -- Dr. Hannes Reinecke zSeries & Storage hare@suse.de +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/