Re: [RFC 3/3] memcg: get rid of mm_struct::owner

Johannes Weiner
2015-05-26T17:20:46Z

On Tue, May 26, 2015 at 05:11:49PM +0200, Michal Hocko wrote:
> On Tue 26-05-15 10:10:11, Johannes Weiner wrote:
> > On Tue, May 26, 2015 at 01:50:06PM +0200, Michal Hocko wrote:
> > > @@ -104,7 +105,12 @@ static inline bool mm_match_cgroup(struct mm_struct *mm,
> > > bool match = false;
> > >
> > > rcu_read_lock();
> > > - task_memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> > > + /*
> > > + * rcu_dereference would be better but mem_cgroup is not a complete
> > > + * type here
> > > + */
> > > + task_memcg = READ_ONCE(mm->memcg);
> > > + smp_read_barrier_depends();
> > > if (task_memcg)
> > > match = mem_cgroup_is_descendant(task_memcg, memcg);
> > > rcu_read_unlock();
> >
> > This function has only one user in rmap. If you inline it there, you
> > can use rcu_dereference() and get rid of the specialness & comment.
>
> I am not sure I understand. struct mem_cgroup is defined in
> mm/memcontrol.c so mm/rmap.c will not see it. Or do you suggest pulling
> struct mem_cgroup out into a header with all the dependencies?

Yes, I think that would be preferrable. It's weird that we have such
a major data structure that is used all over the mm-code but only in
the shape of pointers to an incomplete type. It forces a bad style of
code that uses uninlinable callbacks and accessors for even the most
basic things. There are a few functions in memcontrol.c that could
instead be static inlines or should even be implemented as part of the
code that is using them, such as mem_cgroup_get_lru_size(),
mem_cgroup_is_descendant, mem_cgroup_inactive_anon_is_low(),
mem_cgroup_lruvec_online(), mem_cgroup_swappiness(),
mem_cgroup_select_victim_node(), mem_cgroup_update_page_stat(), and
mem_cgroup_events(). Your new functions fall into the same category.

> @@ -486,29 +486,13 @@ void mm_set_memcg(struct mm_struct *mm, struct mem_cgroup *memcg)
> void mm_drop_memcg(struct mm_struct *mm)
> {
> /*
> - * This is the last reference to mm so nobody can see
> - * this memcg
> + * We could reset mm->memcg, but the mm goes away as this is the
> + * last reference.
> */
> if (mm->memcg)
> css_put(&mm->memcg->css);
> }

This function is supposed to be an API call to disassociate a mm from
its memcg, but it actually doesn't do that and will leave a dangling
pointer based on assumptions it makes about how and when the caller
invokes it. That's bad. It's a subtle optimization with dependencies
spread across two moving parts. The result is very fragile code which
will break things in non-obvious ways when the caller changes later on.

And what's left standing is silly too: a memcg-specific API to call
css_put(), even though struct cgroup_subsys_state and css_put() are
public API already.

Both these things are a negative side effect of struct mem_cgroup
being semi-private. Memcg pointers are everywhere, yet we need a
public interface indirection for every simple dereference.

> @@ -5252,10 +5236,15 @@ static void mem_cgroup_move_task(struct cgroup_subsys_state *css,
>
> if (mm) {
> /*
> - * Commit to a new memcg. mc.to points to the destination
> - * memcg even when the current charges are not moved.
> + * Commit to the target memcg even when we do not move
> + * charges.
> */
> - mm_move_memcg(mm, mc.to);
> + struct mem_cgroup *old_memcg = READ_ONCE(mm->memcg);
> + struct mem_cgroup *new_memcg = mem_cgroup_from_css(css);
> +
> + mm_set_memcg(mm, new_memcg);
> + if (old_memcg)
> + css_put(&old_memcg->css);

"Commit" is a problematic choice of words because of its existing
meaning in memcg of associating a page with a pre-reserved charge.

I'm not sure a comment is actually necessary here. Reassigning
mm->memcg when moving a process pretty straight forward IMO.