Re: [RFC][PATCH 3/7] memcg: use css_get/put when charging/uncharging kmem

Michal Hocko
2013-04-05T13:48:44Z

On Fri 05-04-13 14:19:30, Glauber Costa wrote:
>
> > * __mem_cgroup_free will issue static_key_slow_dec because this
> > * memcg is active already. If the later initialization fails
> > * then the cgroup core triggers the cleanup so we do not have
> > * to do it here.
> > */
> >> - mem_cgroup_get(memcg);
> >> static_key_slow_inc(&memcg_kmem_enabled_key);
> >>
> >> mutex_lock(&set_limit_mutex);
> >> @@ -5823,23 +5814,33 @@ static int memcg_init_kmem(struct mem_cgroup *memcg, struct cgroup_subsys *ss)
> >> return mem_cgroup_sockets_init(memcg, ss);
> >> };
> >>
> >> -static void kmem_cgroup_destroy(struct mem_cgroup *memcg)
> >> +static void kmem_cgroup_css_offline(struct mem_cgroup *memcg)
> >> {
> >> - mem_cgroup_sockets_destroy(memcg);
> >> + /*
> >> + * kmem charges can outlive the cgroup. In the case of slab
> >> + * pages, for instance, a page contain objects from various
> >> + * processes, so it is unfeasible to migrate them away. We
> >> + * need to reference count the memcg because of that.
> >> + */
> >
> > I would prefer if we could merge all three comments in this function
> > into a single one. What about something like the following?
> > /*
> > * kmem charges can outlive the cgroup. In the case of slab
> > * pages, for instance, a page contain objects from various
> > * processes. As we prevent from taking a reference for every
> > * such allocation we have to be careful when doing uncharge
> > * (see memcg_uncharge_kmem) and here during offlining.
> > * The idea is that that only the _last_ uncharge which sees
> > * the dead memcg will drop the last reference. An additional
> > * reference is taken here before the group is marked dead
> > * which is then paired with css_put during uncharge resp. here.
> > * Although this might sound strange as this path is called when
> > * the reference has already dropped down to 0 and shouldn't be
> > * incremented anymore (css_tryget would fail) we do not have
> > * other options because of the kmem allocations lifetime.
> > */
> >> + css_get(&memcg->css);
> >
> > I think that you need a write memory barrier here because css_get
> > nor memcg_kmem_mark_dead implies it. memcg_uncharge_kmem uses
> > memcg_kmem_test_and_clear_dead which imply a full memory barrier but it
> > should see the elevated reference count. No?
> >
>
> We don't use barriers for any other kind of reference counting. What is
> different here?

Now we need to make sure that the racing uncharge sees an elevated
reference count before the group is marked dead. Otherwise we could see
a dead group with ref count == 0, no?

--
Michal Hocko
SUSE Labs