Messages in this thread | | | Date | Thu, 4 Aug 2011 12:20:39 +0200 | From | Tejun Heo <> | Subject | Re: [patch] blk-flush: fix flush policy calculation |
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Hello,
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: ... > 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.
Ah, okay, what changed there was where a request is passed into flush machinery. Before, it was while the request was being dispatched from elevator to device. After, it's de-composed when the request enters elevator. The bug is that there are paths which insert new requests to elevator but didn't check for REQ_FLUSH|FUA.
I think it would be cleaner to add a wrapper around __elv_add_request() which checks for REQ_FLUSH|FUA and enforce REQ_INSERT_FLUSH if the request needs it. Note that this should only happen when a request enters the queue for the first time but not on requeues - that was the reason why the decision wasn't made inside __elv_add_request().
Thank you.
-- tejun
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