On Sat, 17 Dec 2016, Mike Snitzer wrote:

On Fri, Dec 16 2016 at  5:29pm -0500,
Eric Wheeler [off-list ref] wrote:

quoted

On Wed, 14 Dec 2016, Eric Wheeler wrote:

quoted

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.

quoted

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