[patch 2/4] mm: memcontrol: don't pass a NULL memcg to mem_cgroup_end_move()

[Johannes Weiner <hannes@cmpxchg.org>]

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>
---
 mm/memcontrol.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1ff125d2a427..c1fe774d712a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1452,14 +1452,8 @@ 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);
-		atomic_dec(&memcg->moving_account);
-	}
+	atomic_dec(&memcg_moving);
+	atomic_dec(&memcg->moving_account);
 }
 
 /*
@@ -5383,7 +5377,8 @@ 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,
-- 
2.1.2

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