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@xxxxxxx>
---
mm/memcontrol.c | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 0e52ec9..1f5528d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1077,12 +1077,9 @@ 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;
}
@@ -1100,7 +1097,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
spin_lock(&iter->iter_lock);
if (prev && reclaim->generation != iter->generation) {
spin_unlock(&iter->iter_lock);
- return NULL;
+ goto out_css_put;
}
id = iter->position;
}
@@ -1124,8 +1121,12 @@ 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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