On Tue, Aug 17 2010 at 5:33am -0400, Tejun Heo <tj@xxxxxxxxxx> wrote: > 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? OK, I missed this info because I just looked at the DM patch. > >> 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. Makes sense, but your patches still need to be refreshed against the latest (2.6.36-rc1) upstream code. Numerous changes went in to DM recently. > >> * 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. I don't buy it.. you're far from lazy! ;) > >> 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. You'll need Mikulas (bio-based) and NEC (request-based, Kiyoshi and Jun'ichi) to give it serious review. NOTE: NEC has already given some preliminary feedback to hch in the "[PATCH, RFC 2/2] dm: support REQ_FLUSH directly" thread: https://www.redhat.com/archives/dm-devel/2010-August/msg00026.html https://www.redhat.com/archives/dm-devel/2010-August/msg00033.html > > 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. Thanks for the additional details. > * 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? OK, so you folded dm_flush() directly into process_flush() -- the code that was dm_flush() only needs to be called once now. As for your specific dm_wait_for_completion() concern -- I'll defer to Mikulas. But I'll add: we haven't had any reported starvation issues with DM's existing barrier support. DM uses a mempool for its clones, so it should naturally throttle (without starvation) when memory gets low. > * 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). bio-based DM already split the barrier out from the data (in process_barrier). You've renamed process_barrier to process_flush and added the REQ_FLUSH logic like I'd expect. > * 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. OK, so seems 1 is done, 2 is still TODO. Looking at your tree it seems 2 would be as simple as using the following in dm_init_request_based_queue (on the most current upstream dm.c): blk_queue_flush(q, REQ_FLUSH | REQ_FUA); (your current patch only sets REQ_FLUSH in alloc_dev). -- 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
