[PATCH v4 1/6] memcg: keep prev's css alive for the whole mem_cgroup_iter

[Michal Hocko <mhocko@suse.cz>]

css reference counting keeps the cgroup alive even though it has been
already removed. mem_cgroup_iter relies on this fact and takes a
reference to the returned group. The reference is then released on the
next iteration or mem_cgroup_iter_break.
mem_cgroup_iter currently releases the reference right after it gets the
last css_id.
This is correct because neither prev's memcg nor cgroup are accessed
after then. This will change in the next patch so we need to hold the
group alive a bit longer so let's move the css_put at the end of the
function.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
 mm/memcontrol.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1ea8951..f3d1bfe 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1141,12 +1141,9 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 	if (prev && !reclaim)
 		id = css_id(&prev->css);
 
-	if (prev && prev != root)
-		css_put(&prev->css);
-
 	if (!root->use_hierarchy && root != root_mem_cgroup) {
 		if (prev)
-			return NULL;
+			goto out_css_put;
 		return root;
 	}
 
@@ -1162,7 +1159,7 @@ 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)
-				return NULL;
+				goto out_css_put;
 			id = iter->position;
 		}
 
@@ -1184,8 +1181,12 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
 		}
 
 		if (prev && !css)
-			return NULL;
+			goto out_css_put;
 	}
+out_css_put:
+	if (prev && prev != root)
+		css_put(&prev->css);
+
 	return memcg;
 }
 
-- 
1.7.10.4

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