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

From: Michal Hocko
Date: Wed Oct 17 2012 - 09:32:30 EST

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;
ret = mem_cgroup_force_empty(memcg);

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