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>

1 files changed, 3 insertions, 7 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a5c9aa4688e8..3cd4f1e0bfb3 100644
--- 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,