Re: [BUG] NULL pointer de-ref when setting io.cost.qos on LUKS devices

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

Hello,

On Thu, Mar 05, 2020 at 11:31:29AM +0100, Benjamin Berg wrote:
> > In the longer term, what we wanna do is controlling at physical
> > devices (sda here) and then updating dm so that it can maintain and
> > propagate the ownership correctly but we aren't there yet.
> 
> Perfect, so what I am seeing is really just a small systemd bug. Thansk
> for confirming, I'll submit a patch to fix it.

IO control is a bit confusing right now. Here's the breakdown.

* There are four controllers - io.latency, io.cost, io.max and bfq's
  weight implementation.

* io.latency and io.cost when combined with btrfs can control all IOs
  including metadata IOs and writebacks while avoiding priority
  inversions.

* wbt may interfere with IO control. It can be disabled with "echo 0 >
  /sys/block/DEV/queue/wbt_lat_usec".

* io.latency is useful to protect one thing against everything else
  but it gets tricky when multiple entities competing at different
  priority levels.

* io.cost is what we're verifying against and deploying. While
  system-level configuration is a bit involved
  (/sys/fs/cgroup/io.cost.model and /sys/fs/cgroup/io.cost.qos).
  Actual cgroup configuration is really simple. Simply enabling IO
  controller and leaving all weights at default often can achieve most
  of what's needed.

Thanks.

-- 
tejun