Re: [RFC] block/loop: make loop cgroup aware

[Tejun Heo <tj@kernel.org>]

Hello, Shaohua.

On Tue, Aug 29, 2017 at 08:22:36AM -0700, Shaohua Li wrote:
> > Yeah, writeback tracks the most active cgroup and associates writeback
> > ios with that cgroup.  For buffered loop devices, I think it'd be fine
> > to make the loop driver forward the cgroup association and let the
> > writeback layer determine the matching association as usual.
> 
> Doing this means we must forward cgroup info to page, not bio. I need to check
> if we can make the mechanism work for memcg.

The association is already coming from the page.  We just need to make
sure that going through loop driver doesn't get in the way of the
membership getting propagated to the underlying device.

> > Hmm... why do we need double forwarding (original issuer -> aio cmd ->
> > ios) in loop?  If we need this, doesn't this mean that we're missing
> > ownership forwarding in aio paths and should make the forwarding
> > happen there?
> 
> what do you mean double forwarding?

So, this looks like the loop driver is explicitly forwarding the
association from the original issuer to the aio command and then from
the aio command to the ios to the underlying device.  I'm wondering
whether the right way to do this is making aio forward the association
by default, instead of the loop driver doing it explicitly.  That way,
all aio users can benefit from the forwarding instead of just loop.

Thanks.

-- 
tejun