From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: Block regression since 3.1-rc3 Date: Sat, 8 Oct 2011 12:14:21 -0400 Message-ID: <20111008161421.GA5743@redhat.com> References: <1317397918.27140.15.camel@localhost> <1317729761.25998.4.camel@localhost> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Shaohua Li Cc: Jeff Moyer , Jens Axboe , Tejun Heo , device-mapper development , Christophe Saout , linux-kernel@vger.kernel.org List-Id: dm-devel.ids On Sat, Oct 08 2011 at 7:02am -0400, Shaohua Li wrote: > Looks the dm request based flush logic is broken. > > saved_make_request_fn > __make_request > blk_insert_flush > but blk_insert_flush doesn't put the original request to list, instead, the > q->flush_rq is in list. > then > dm_request_fn > blk_peek_request > dm_prep_fn > clone_rq > map_request > blk_insert_cloned_request > so q->flush_rq is cloned, and get dispatched. but we can't clone q->flush_rq > and use it to do flush. map_request even could assign a different blockdev to > the cloned request. You haven't explained why cloning q->flush_rq is broken. What is the problem with map_request changing the blockdev? For the purposes of request-based DM the flush machinery has already managed the processing of the flush at the higher level request_queue. By the time request-based DM is cloning a flush request it really has no need to reenter the flush machinery (even though Tejun wants it to -- but in practice it doesn't buy us anything because we never stack request-based DM at the moment. Instead it showcases how brittle this path is). > Clone q->flush_rq is absolutely wrong. I'm still missing the _why_. Taking a step back: Unless others have an immediate ah-ha moment, I'd suggest we revert commit 4853abaae7e4a2a (block: fix flush machinery for stacking drivers with differring flush flags). Whereby avoiding unnecessarily reentering the flush machinery. If commit ed8b752bccf256 (dm table: set flush capability based on underlying devices) is in place the flush gets fed directly to scsi_request_fn, which is fine because the request-based DM's request_queue's flush_flags reflect the flush capabilities of the underlying device(s). We are then covered relative to the only request-based DM use-case people care about (e.g. dm-multipath, which doesn't use stacked request-based DM). We can revisit upholding the purity of the flush machinery for stacked devices in >= 3.2.