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

[Michal Hocko <mhocko@suse.cz>]

On Tue 26-05-15 13:20:19, Johannes Weiner wrote:
> 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

Fair enough. I was afraid of dependencies between networking and memcg
header files but it seems that only struct cg_proto is really needed for
tcp kmem controller and that one doesn't depend on any socket specific
stuff. So we are good here. 

> 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.

Let me try how this will end up. Hopefully the code will not grow too
much.

> > @@ -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.

Fair point. The optimization is not really worth it and I will add
explicit NULLing because I would prefer to keep the function as well as
mm_set_memcg because this is easier to track and at least mm_set_memcg
needs to be called from two places (as pointed out by Oleg) and I would
really like prevent from duplication.

> 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.

OK, will remove it.
-- 
Michal Hocko
SUSE Labs

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