From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754848Ab1HBSbI (ORCPT ); Tue, 2 Aug 2011 14:31:08 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13522 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753348Ab1HBSbF (ORCPT ); Tue, 2 Aug 2011 14:31:05 -0400 From: Jeff Moyer To: Vivek Goyal Cc: Shaohua Li , linux-kernel@vger.kernel.org, Tejun Heo , Jens Axboe , msnitzer@redhat.com Subject: Re: [patch] blk-flush: fix flush policy calculation References: <20110802181739.GE6399@redhat.com> X-PGP-KeyID: 1F78E1B4 X-PGP-CertKey: F6FE 280D 8293 F72C 65FD 5A58 1FF8 A7CA 1F78 E1B4 X-PCLoadLetter: What the f**k does that mean? Date: Tue, 02 Aug 2011 14:31:00 -0400 In-Reply-To: <20110802181739.GE6399@redhat.com> (Vivek Goyal's message of "Tue, 2 Aug 2011 14:17:39 -0400") Message-ID: User-Agent: Gnus/5.110011 (No Gnus v0.11) Emacs/23.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Vivek Goyal writes: > 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. Right, I still stand by that fix. It was a thinko. > - 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. Good point. > - 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. :-) You missed __elv_next_request. :) > 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. In testing, I did this: @@ -1817,6 +1817,14 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq) return -EIO; #endif + if ((rq->cmd_flags & (REQ_FLUSH|REQ_FUA)) && !q->flush_flags) { + rq->cmd_flags &= ~(REQ_FLUSH|REQ_FUA); + if (!blk_rq_bytes(rq)) { + blk_end_request(rq, 0, 0); + return 0; + } + } + Cheers, Jeff