On Sat, Oct 08 2011 at 7:02am -0400, Shaohua Li <shli@xxxxxxxxxx> 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. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
