mm: memcontrol: don't pass a NULL memcg to mem_cgroup_end_move()

mem_cgroup_end_move() checks if the passed memcg is NULL, along with a
lengthy comment to explain why this seemingly non-sensical situation is
even possible.

Check in cancel_attach() itself whether can_attach() set up the move
context or not, it's a lot more obvious from there.  Then remove the check
and comment in mem_cgroup_end_move().

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a5c9aa4..3cd4f1e 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1469,12 +1469,7 @@ static void mem_cgroup_start_move(struct mem_cgroup *memcg)
 
 static void mem_cgroup_end_move(struct mem_cgroup *memcg)
 {
-       /*
-        * Now, mem_cgroup_clear_mc() may call this function with NULL.
-        * We check NULL in callee rather than caller.
-        */
-       if (memcg)
-               atomic_dec(&memcg->moving_account);
+       atomic_dec(&memcg->moving_account);
 }
 
 /*
@@ -5489,7 +5484,8 @@ static int mem_cgroup_can_attach(struct cgroup_subsys_state *css,
 static void mem_cgroup_cancel_attach(struct cgroup_subsys_state *css,
                                     struct cgroup_taskset *tset)
 {
-       mem_cgroup_clear_mc();
+       if (mc.to)
+               mem_cgroup_clear_mc();
 }
 
 static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,