From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754894Ab1HBSSW (ORCPT ); Tue, 2 Aug 2011 14:18:22 -0400 Received: from mx1.redhat.com ([209.132.183.28]:15401 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754868Ab1HBSSR (ORCPT ); Tue, 2 Aug 2011 14:18:17 -0400 Date: Tue, 2 Aug 2011 14:17:39 -0400 From: Vivek Goyal To: Jeff Moyer Cc: Shaohua Li , linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe , msnitzer@redhat.com Subject: Re: [patch] blk-flush: fix flush policy calculation Message-ID: <20110802181739.GE6399@redhat.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 02, 2011 at 01:39:46PM -0400, Jeff Moyer wrote: > OK, sorry for top-posting here, but I chased the problem down further. > > Commit ae1b1539622fb46e51b4d13b3f9e5f4c713f86ae, block: reimplement > FLUSH/FUA to support merge, introduced a regression when running any > sort of fsyncing workload using dm-multipath and certain storage (in our > case, an HP EVA). It turns out that dm-multipath always advertised > flush+fua support, and passed commands on down the stack, where they > used to get stripped off. The above commit, unfortunately, changed that > behavior: > > static inline struct request *__elv_next_request(struct request_queue *q) > { > struct request *rq; > > while (1) { > - while (!list_empty(&q->queue_head)) { > + if (!list_empty(&q->queue_head)) { > rq = list_entry_rq(q->queue_head.next); > - if (!(rq->cmd_flags & (REQ_FLUSH | REQ_FUA)) || > - (rq->cmd_flags & REQ_FLUSH_SEQ)) > - return rq; > - rq = blk_do_flush(q, rq); > - if (rq) > - return rq; > + return rq; > } > > Note that previously, a command would come in here, have > REQ_FLUSH|REQ_FUA set, and then get handed off to blk_do_flush: > > struct request *blk_do_flush(struct request_queue *q, struct request *rq) > { > unsigned int fflags = q->flush_flags; /* may change, cache it */ > bool has_flush = fflags & REQ_FLUSH, has_fua = fflags & REQ_FUA; > bool do_preflush = has_flush && (rq->cmd_flags & REQ_FLUSH); > bool do_postflush = has_flush && !has_fua && (rq->cmd_flags & > REQ_FUA); > unsigned skip = 0; > ... > if (blk_rq_sectors(rq) && !do_preflush && !do_postflush) { > rq->cmd_flags &= ~REQ_FLUSH; > if (!has_fua) > rq->cmd_flags &= ~REQ_FUA; > return rq; > } > > So, the flush machinery was bypassed in such cases (q->flush_flags == 0 > && rq->cmd_flags & (REQ_FLUSH|REQ_FUA)). > > Now, however, we don't get into the flush machinery at all (which is why > my initial patch didn't help this situation). Instead, > __elv_next_request just hands a request with flush and fua bits set to > the scsi_request_fn, even though the underlying request_queue does not > support flush or fua. > > So, where do we fix this? We could just accept Mike's patch to not send > such requests down from dm-mpath, but that seems short-sighted. We > could reinstate some checks in __elv_next_request. Or, we could put the > checks into blk_insert_cloned_request. > > Suggestions? IMHO, we should fix it at multiple places. - Your initial fix in blk_insert_flush makes sense. blk_insert_flush() is equivalent of blk_do_flush() so resetting REQ_FLUSH and REQ_FUA there makes sense to me. - Fixing blk_insert_cloned_request() also makes sense to me so that if a request is REQ_FLUSH or REQ_FUA set, we try to add it to underlying device using ELEVATOR_INSERT_FLUSH and not ELEVATOR_INSERT_BACK. - Fixing dm-multipath makes sense too as what's the point in dispatching unnecessary flush/fua requests to underlying devices if underlying queue does not have FLUSH capability. So I would say, fix it at all the places. :-) I have one question though. What happens if we have an empty request with REQ_FLUSH set and request queue does not support flush. Where will we complete the IO for that request? I see that __generic_make_request() takes care of that but we might have to take care of if it insert_cloned path too. Thanks Vivek