On Sat, 17 Dec 2016, Mike Snitzer wrote: > On Fri, Dec 16 2016 at 5:29pm -0500, > Eric Wheeler <dm-devel@xxxxxxxxxxxxxxxxxx> wrote: > > On Wed, 14 Dec 2016, Eric Wheeler wrote: > > > Since dm-crypt queues writes (and sometimes reads) to a different kernel > > > thread (workqueue), the bios will dispatch from tasks with different > > > io_context->ioprio settings than the submitting task, thus giving > > > incorrect ioprio hints to the io scheduler. > > > > The motivation of this patch is for ioprio-driven writebackup/bypass > > hinting inside bcache when bcache is under dm-crypt which Jens is picking > > up for 4.10: > > https://lkml.org/lkml/2016/12/6/607 > > I now see your commits: > b71de4659fba4e42c7 bcache: introduce per-process ioprio-based bypass/writeback hints > 82e7192711c3855038 bcache: documentation for ioprio cache hinting > > You'd think this is the type of thing that you'd have proposed to a > wider audience. ( Its not really relevant to this bugfix, but please see this thread since you were curious about a wider audience discussion: https://www.redhat.com/archives/dm-devel/2016-July/msg00556.html ) The note above in the previous email was intended to explain how we discovered the dm-crypt problem, purely as an example use case. The stable commit note discusses the real issue: lost elevator hints. This commit fixes elevator ioprio hints passing through dm-crypt and is not intended to address dm-cache, nor enable a bcache feature. All impementations using ioprio hints beneath dm-crypt would benefit---most importantly, _CFQ_ ! As it is, all ioprio hints passing through dm-crypt are lost to the elevator; the elevator looses those useful bits because of queuing to another thread for crypto operations. > > The ioprio aware schedulers like CFQ and BFQ also benefit with more > > knowledge about the bio's when passing through dm-crypt. It would be > > great if this can be accepted for 4.10, too. > > It also needs more review, testing and possible re-working. Each DM > target shouldn't have to worry about these details (though I do grant > that dm-crypt.c:clone_init call to bio_set_prio makes sense). The patch is trivial: +5 lines in dm-crypt.c (excluding `if` bracing), a helper function in bio.h, and a 1-line fixup in bio.c. Thus, the stable@ inclusion since it would probably patch down to v3.x somewhere and help everyone who uses dm-crypt with ionice. Its only 4.10-rc1, and as a bugfix it could be accepted as a stable commit for 4.10-rc2 or later if you are willing to help dm-crypt users who use ionice. Indeed, if you so choose, you could both accept this commit and still re-work it in 4.11. This is to benefit everyone, using kernels both old and new. Cheers, -Eric -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
