[PATCH 2/6] memcg: root_cgroup cannot reach mem_cgroup_move_parent

[Michal Hocko]

The root cgroup cannot be destroyed so we never hit it idown the
mem_cgroup_pre_destroy path and mem_cgroup_force_empty_write shouldn't
even try to do anything if called for the root.

This means that mem_cgroup_move_parent doesn't have to bother with the
root cgroup and it can assume it can always move charges upwards.

Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
---
 mm/memcontrol.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f25e9c0..9ce24b7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2709,9 +2709,7 @@ static int mem_cgroup_move_parent(struct page *page,
 	unsigned long uninitialized_var(flags);
 	int ret;
 
-	/* Is ROOT ? */
-	if (mem_cgroup_is_root(child))
-		return -EINVAL;
+	VM_BUG_ON(mem_cgroup_is_root(child));
 
 	ret = -EBUSY;
 	if (!get_page_unless_zero(page))
@@ -3817,6 +3815,8 @@ static int mem_cgroup_force_empty_write(struct cgroup *cont, unsigned int event)
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
 	int ret;
 
+	if (mem_cgroup_is_root(memcg))
+		return -EINVAL;
 	css_get(&memcg->css);
 	ret = mem_cgroup_force_empty(memcg);
 	css_put(&memcg->css);
-- 
1.7.10.4

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