Hello, On 08/16/2010 09:02 PM, Mike Snitzer wrote: > On Mon, Aug 16 2010 at 12:52pm -0400, > Tejun Heo <tj@xxxxxxxxxx> wrote: > >> From: Tejun Heo <tj@xxxxxxxxxx> >> >> This patch converts dm to support REQ_FLUSH/FUA instead of now >> deprecated REQ_HARDBARRIER. > > What tree does this patch apply to? I know it doesn't apply to > v2.6.36-rc1, e.g.: http://git.kernel.org/linus/708e929513502fb0 (from the head message) These patches are on top of block#for-2.6.36-post (c047ab2dddeeafbd6f7c00e45a13a5c4da53ea0b) + block-replace-barrier-with-sequenced-flush patchset[1] + block-fix-incorrect-bio-request-flag-conversion-in-md patch[2] and available in the following git tree. git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git flush-fua [1] http://thread.gmane.org/gmane.linux.kernel/1022363 [2] http://thread.gmane.org/gmane.linux.kernel/1023435 Probably fetching the git tree is the easist way to review? >> For bio-based dm, >> * -EOPNOTSUPP retry logic dropped. > > That logic wasn't just about retries (at least not in the latest > kernel). With commit 708e929513502fb0 the -EOPNOTSUPP checking also > serves to optimize the barrier+discard case (when discards aren't > supported). With the patch applied, there's no second flush. Those requests would now be REQ_FLUSH + REQ_DISCARD. The first can't be avoided anyway and there won't be the second flush to begin with, so I don't think this worsens anything. >> * Nothing much changes. It just needs to handle FLUSH requests as >> before. It would be beneficial to advertise FUA capability so that >> it can propagate FUA flags down to member request_queues instead of >> sequencing it as WRITE + FLUSH at the top queue. > > Can you expand on that TODO a bit? What is the mechanism to propagate > FUA down to a DM device's members? I'm only aware of propagating member > devices' features up to the top-level DM device's request-queue (not the > opposite). > > Are you saying that establishing the FUA capability on the top-level DM > device's request_queue is sufficient? If so then why not make the > change? Yeah, I think it would be enough to always advertise FLUSH|FUA if the member devices support FLUSH (regardless of FUA support). The reason why I didn't do it was, umm, laziness, I suppose. >> Lightly tested linear, stripe, raid1, snap and crypt targets. Please >> proceed with caution as I'm not familiar with the code base. > > This is concerning... Yeap, I want you to be concerned. :-) This was the first time I looked at the dm code and there are many different disjoint code paths and I couldn't fully follow or test all of them, so it definitely needs a careful review from someone who understands the whole thing. > if we're to offer more comprehensive review I think we need more > detail on what guided your changes rather than details of what the > resulting changes are. I'll try to explain it. If you have any further questions, please let me know. * For common part (dm-io, dm-log): * Empty REQ_HARDBARRIER is converted to empty REQ_FLUSH. * REQ_HARDBARRIER w/ data is converted either to data + REQ_FLUSH + REQ_FUA or data + REQ_FUA. The former is the safe equivalent conversion but there could be cases where ther latter is enough. * For bio based dm: * Unlike REQ_HARDBARRIER, REQ_FLUSH/FUA doesn't have any ordering requirements. Remove assumptions of ordering and/or draining. A related question: Is dm_wait_for_completion() used in process_flush() safe against starvation under continuous influx of other commands? * As REQ_FLUSH/FUA doesn't require any ordering of requests before or after it, on array devices, the latter part - REQ_FUA - can be handled like other writes. ie. REQ_FLUSH needs to be broadcasted to all devices but once that is complete the data/REQ_FUA bio can be sent to only the affected devices. This needs some care as there are bio cloning/splitting code paths where REQ_FUA bit isn't preserved. * Guarantee that REQ_FLUSH w/ data never reaches targets (this in part is to put it in alignment with request based dm). * For request based dm: * The sequencing is done by the block layer for the top level request_queue, so the only things request based dm needs to make sure is 1. handling empty REQ_FLUSH correctly (block layer will only send down empty REQ_FLUSHes) and 2. propagate REQ_FUA bit to member devices. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
