Re: [PATCH 7/8] memcg: get rid of mm_struct::owner

Vladimir Davydov
2015-07-10T07:54:21Z

On Thu, Jul 09, 2015 at 04:09:41PM +0200, Michal Hocko wrote:
> On Wed 08-07-15 20:32:51, Vladimir Davydov wrote:
> > On Wed, Jul 08, 2015 at 02:27:51PM +0200, Michal Hocko wrote:
[...]
> > > @@ -474,7 +519,7 @@ static inline void mem_cgroup_count_vm_event(struct mm_struct *mm,
> > > return;
> > >
> > > rcu_read_lock();
> > > - memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
> > > + memcg = rcu_dereference(mm->memcg);
> > > if (unlikely(!memcg))
> > > goto out;
> > >
> >
> > If I'm not mistaken, mm->memcg equals NULL for any task in the root
> > memory cgroup
>
> right
>
> > (BTW, it it's true, it's worth mentioning in the comment
> > to mm->memcg definition IMO). As a result, we won't account the stats
> > for such tasks, will we?
>
> well spotted! This is certainly a bug. There are more places which are
> checking for mm->memcg being NULL and falling back to root_mem_cgroup. I
> think it would be better to simply use root_mem_cgroup right away. We
> can setup init_mm.memcg = root_mem_cgroup during initialization and be
> done with it. What do you think? The diff is in the very end of the
> email (completely untested yet).

I'd prefer initializing init_mm.memcg to root_mem_cgroup. This way we
wouldn't have to check whether mm->memcg is NULL or not here and there,
which would make the code cleaner IMO.

[...]
> > > @@ -4932,14 +4943,26 @@ static void mem_cgroup_move_task(struct cgroup_subsys_state *css,
> > > {
> > > struct task_struct *p = cgroup_taskset_first(tset);
> > > struct mm_struct *mm = get_task_mm(p);
> > > + struct mem_cgroup *old_memcg = NULL;
> > >
> > > if (mm) {
> > > + old_memcg = READ_ONCE(mm->memcg);
> > > + __mm_set_memcg(mm, mem_cgroup_from_css(css));
> > > +
> > > if (mc.to)
> > > mem_cgroup_move_charge(mm);
> > > mmput(mm);
> > > }
> > > if (mc.to)
> > > mem_cgroup_clear_mc();
> > > +
> > > + /*
> > > + * Be careful and drop the reference only after we are done because
> > > + * p's task_css memcg might be different from p->memcg and nothing else
> > > + * might be pinning the old memcg.
> > > + */
> > > + if (old_memcg)
> > > + css_put(&old_memcg->css);
> >
> > Please explain why the following race is impossible:
> >
> > CPU0 CPU1
> > ---- ----
> > [current = T]
> > dup_mm or exec_mmap
> > mm_inherit_memcg
> > memcg = current->mm->memcg;
> > mem_cgroup_move_task
> > p = T;
> > mm = get_task_mm(p);
> > old_memcg = mm->memcg;
> > css_put(&old_memcg->css);
> > /* old_memcg can be freed now */
> > css_get(memcg); /* BUG */
>
> I guess you are right. The window seem to be very small but CPU0 simly
> might get preempted by the moving task and so even cgroup pinning
> wouldn't help here.
>
> I guess we need
> ---
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index b3e7e30b5a74..6fbd33273b6d 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -300,9 +300,17 @@ void __mm_set_memcg(struct mm_struct *mm, struct mem_cgroup *memcg)
> static inline
> void mm_inherit_memcg(struct mm_struct *newmm, struct mm_struct *oldmm)
> {
> - struct mem_cgroup *memcg = oldmm->memcg;
> + struct mem_cgroup *memcg;
>
> + /*
> + * oldmm might be under move and just replacing its memcg (see
> + * mem_cgroup_move_task) so we have to protect from its memcg
> + * going away between we dereference and take a reference.
> + */
> + rcu_read_lock();
> + memcg = rcu_dereference(oldmm->memcg);
> __mm_set_memcg(newmm, memcg);

If it's safe to call css_get under rcu_read_lock, then it's OK,
otherwise we probably need to use a do {} while (!css_tryget(memcg))
loop in __mm_set_memcg.

> + rcu_read_unlock();
> }
>
> /**
>
>
> Make sure that all tasks have non NULL memcg.
[...]

That looks better to me.

Thanks,
Vladimir