(2012/11/14 19:10), Michal Hocko wrote:
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);
I think this will work.
Thanks,
-Kame
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