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

[Tejun Heo <tj@kernel.org>]

Hello, Shaohua.

On Tue, Aug 29, 2017 at 10:07:04PM -0700, Shaohua Li wrote:
> > 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.
> 
> I think there is confusion. App writes files in upper layer fs on loop. memcg

Ah, yes, for some reason, I was still thinking about direct ios.

> estabilish membership for the pages of these files. Then writeback does write,
> loop then write these pages to under layer fs. The write is done in loop

Yes.

> thread. The write will allocate new page cache for under layer fs files. The
> issue is we must forward memcg info from upper layer files page cache to under
> layer files page cache.

Ah, I see.  We need to assign the ownership of the page cache pages to
the original dirtier.  Yeah, that's a different problem.  For now,
let's concentrate on the dio case.

> > 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.
> 
> That's possible. The downside doing this in aio is we must audit all fs to make
> sure all bio have association. I'd like to avoid doing this if there is no
> other loop-like cgroup usage.

But wouldn't that mean that we break cgroup membership for aios that
users issue?  Can't we do this in the generic aio layer?

Thanks.

-- 
tejun