Re: [PATCH v6 05/19] mm: memcontrol: decouple reference counting from page accounting

Shakeel Butt
2020-06-19T01:32:04Z

On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@fb.com> wrote:
>
> From: Johannes Weiner <hannes@cmpxchg.org>
>
> The reference counting of a memcg is currently coupled directly to how
> many 4k pages are charged to it. This doesn't work well with Roman's
> new slab controller, which maintains pools of objects and doesn't want
> to keep an extra balance sheet for the pages backing those objects.
>
> This unusual refcounting design (reference counts usually track
> pointers to an object) is only for historical reasons: memcg used to
> not take any css references and simply stalled offlining until all
> charges had been reparented and the page counters had dropped to
> zero. When we got rid of the reparenting requirement, the simple
> mechanical translation was to take a reference for every charge.
>
> More historical context can be found in commit e8ea14cc6ead ("mm:
> memcontrol: take a css reference for each charged page"),
> commit 64f219938941 ("mm: memcontrol: remove obsolete kmemcg pinning
> tricks") and commit b2052564e66d ("mm: memcontrol: continue cache
> reclaim from offlined groups").
>
> The new slab controller exposes the limitations in this scheme, so
> let's switch it to a more idiomatic reference counting model based on
> actual kernel pointers to the memcg:
>
> - The per-cpu stock holds a reference to the memcg its caching
>
> - User pages hold a reference for their page->mem_cgroup. Transparent
> huge pages will no longer acquire tail references in advance, we'll
> get them if needed during the split.
>
> - Kernel pages hold a reference for their page->mem_cgroup
>
> - Pages allocated in the root cgroup will acquire and release css
> references for simplicity. css_get() and css_put() optimize that.
>
> - The current memcg_charge_slab() already hacked around the per-charge
> references; this change gets rid of that as well.
>
> Roman:
> 1) Rebased on top of the current mm tree: added css_get() in
> mem_cgroup_charge(), dropped mem_cgroup_try_charge() part
> 2) I've reformatted commit references in the commit log to make
> checkpatch.pl happy.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Roman Gushchin <guro@fb.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>