[dm-devel] [PATCH] dm crypt: fix lost ioprio when queuing crypto bios from task with ioprio

[Christoph Hellwig]

On Thu, Dec 06, 2018 at 05:15:07PM -0500, Mike Snitzer wrote:
> From: Eric Wheeler <dm-devel at lists.ewheeler.net>
> 
> 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.
> 
> By assigning the ioprio to the bio before queuing to a workqueue, the
> original submitting task's io_context->ioprio setting can be retained
> through the life of the bio.  We only set the bio's ioprio in dm-crypt
> if not already set (by somewhere else, higher in the stack).

The scheme looks a little odd to me.  Instead of requring a driver
call why don't we just make sure bios always have the submitting
tasks priority set and then make sure the lower layers can rely on it?