On Fri 15-02-13 17:03:09, KAMEZAWA Hiroyuki wrote:
[...]
> > @@ -1158,31 +1161,74 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >
> > mz = mem_cgroup_zoneinfo(root, nid, zid);
> > iter = &mz->reclaim_iter[reclaim->priority];
> > - if (prev && reclaim->generation != iter->generation)
> > - goto out_css_put;
> > - id = iter->position;
> > + spin_lock(&iter->iter_lock);
> > + last_visited = iter->last_visited;
> > + if (prev && reclaim->generation != iter->generation) {
> > + if (last_visited) {
> > + css_put(&last_visited->css);
> > + iter->last_visited = NULL;
> > + }
> > + spin_unlock(&iter->iter_lock);
> > + goto out_unlock;
> > + }
[...]
> > + /*
> > + * Even if we found a group we have to make sure it is alive.
> > + * css && !memcg means that the groups should be skipped and
> > + * we should continue the tree walk.
> > + * last_visited css is safe to use because it is protected by
> > + * css_get and the tree walk is rcu safe.
> > + */
> > + if (css == &root->css || (css && css_tryget(css)))
> > + memcg = mem_cgroup_from_css(css);
> >
> > if (reclaim) {
> > - iter->position = id;
> > + struct mem_cgroup *curr = memcg;
> > +
> > + if (last_visited)
> > + css_put(&last_visited->css);
> > +
> > + if (css && !memcg)
> > + curr = mem_cgroup_from_css(css);
> > +
> > + /* make sure that the cached memcg is not removed */
> > + if (curr)
> > + css_get(&curr->css);
> I'm sorry if I miss something...
>
> This curr is curr == memcg = mem_cgroup_from_css(css) <= already try_get() done.
> double refcounted ?
Yes we get 2 references here. One for the returned memcg - which will be
released either by mem_cgroup_iter_break or a next iteration round
(where it would be prev) and the other is for last_visited which is
released when a new memcg is cached.
[...]
--
Michal Hocko
SUSE Labs
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