Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators

[Michal Hocko]

On Wed 14-11-12 09:20:03, KAMEZAWA Hiroyuki wrote:
> (2012/11/14 0:30), Michal Hocko wrote:
[...]
> > @@ -1096,30 +1096,64 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> >   			mz = mem_cgroup_zoneinfo(root, nid, zid);
> >   			iter = &mz->reclaim_iter[reclaim->priority];
> >   			spin_lock(&iter->iter_lock);
> > +			last_visited = iter->last_visited;
> >   			if (prev && reclaim->generation != iter->generation) {
> > +				if (last_visited) {
> > +					mem_cgroup_put(last_visited);
> > +					iter->last_visited = NULL;
> > +				}
> >   				spin_unlock(&iter->iter_lock);
> >   				return NULL;
> >   			}
> > -			id = iter->position;
> >   		}
> >   
> >   		rcu_read_lock();
> > -		css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
> > -		if (css) {
> > -			if (css == &root->css || css_tryget(css))
> > -				memcg = mem_cgroup_from_css(css);
> > -		} else
> > -			id = 0;
> > -		rcu_read_unlock();
> > +		/*
> > +		 * Root is not visited by cgroup iterators so it needs a special
> > +		 * treatment.
> > +		 */
> > +		if (!last_visited) {
> > +			css = &root->css;
> > +		} else {
> > +			struct cgroup *next_cgroup;
> > +
> > +			next_cgroup = cgroup_next_descendant_pre(
> > +					last_visited->css.cgroup,
> > +					root->css.cgroup);
> 
> Maybe I miss something but.... last_visited is holded by memcg's refcnt.
> The cgroup pointed by css.cgroup is by cgroup's refcnt which can be freed
> before memcg is freed and last_visited->css.cgroup is out of RCU cycle.
> Is this safe ?

Good spotted. You are right. What I need to do is to check that the
last_visited is alive and restart from the root if not. Something like
the bellow (incremental patch on top of this one) should help, right?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 30efd7e..c0a91a3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1105,6 +1105,16 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 				spin_unlock(&iter->iter_lock);
 				return NULL;
 			}
+			/*
+			 * memcg is still valid because we hold a reference but
+			 * its cgroup might have vanished in the meantime so
+			 * we have to double check it is alive and restart the
+			 * tree walk otherwise.
+			 */
+			if (last_visited && !css_tryget(&last_visited->css)) {
+				mem_cgroup_put(last_visited);
+				last_visited = NULL;
+			}
 		}
 
 		rcu_read_lock();
@@ -1136,8 +1146,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		if (reclaim) {
 			struct mem_cgroup *curr = memcg;
 
-			if (last_visited)
+			if (last_visited) {
+				css_put(&last_visited->css);
 				mem_cgroup_put(last_visited);
+			}
 
 			if (css && !memcg)
 				curr = mem_cgroup_from_css(css);
-- 
Michal Hocko
SUSE Labs
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