1 files changed, 19 insertions, 13 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ae0433885b69..8e0988fee888 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -416,8 +416,8 @@ static bool memcg_kmem_test_and_clear_dead(struct mem_cgroup *memcg)
 
 /* Stuffs for move charges at task migration. */
 /*
- * Types of charges to be moved. "move_charge_at_immitgrate" is treated as a
- * left-shifted bitmap of these types.
+ * Types of charges to be moved. "move_charge_at_immitgrate" and
+ * "immigrate_flags" are treated as a left-shifted bitmap of these types.
  */
 enum move_type {
         MOVE_CHARGE_TYPE_ANON,  /* private anonymous page and swap of it */
@@ -430,6 +430,7 @@ static struct move_charge_struct {
         spinlock_t        lock; /* for from, to */
         struct mem_cgroup *from;
         struct mem_cgroup *to;
+        unsigned long immigrate_flags;
         unsigned long precharge;
         unsigned long moved_charge;
         unsigned long moved_swap;
@@ -442,14 +443,12 @@ static struct move_charge_struct {
 
 static bool move_anon(void)
 {
-        return test_bit(MOVE_CHARGE_TYPE_ANON,
-                                        &mc.to->move_charge_at_immigrate);
+        return test_bit(MOVE_CHARGE_TYPE_ANON, &mc.immigrate_flags);
 }
 
 static bool move_file(void)
 {
-        return test_bit(MOVE_CHARGE_TYPE_FILE,
-                                        &mc.to->move_charge_at_immigrate);
+        return test_bit(MOVE_CHARGE_TYPE_FILE, &mc.immigrate_flags);
 }
 
 /*
@@ -5193,15 +5192,14 @@ static int mem_cgroup_move_charge_write(struct cgroup *cgrp,
 
         if (val >= (1 << NR_MOVE_TYPE))
                 return -EINVAL;
+
         /*
-         * We check this value several times in both in can_attach() and
-         * attach(), so we need cgroup lock to prevent this value from being
-         * inconsistent.
+         * No kind of locking is needed in here, because ->can_attach() will
+         * check this value once in the beginning of the process, and then carry
+         * on with stale data. This means that changes to this value will only
+         * affect task migrations starting after the change.
          */
-        cgroup_lock();
         memcg->move_charge_at_immigrate = val;
-        cgroup_unlock();
-
         return 0;
 }
 #else
@@ -6559,8 +6557,15 @@ static int mem_cgroup_can_attach(struct cgroup *cgroup,
         struct task_struct *p = cgroup_taskset_first(tset);
         int ret = 0;
         struct mem_cgroup *memcg = mem_cgroup_from_cont(cgroup);
+        unsigned long move_charge_at_immigrate;
 
-        if (memcg->move_charge_at_immigrate) {
+        /*
+         * We are now commited to this value whatever it is. Changes in this
+         * tunable will only affect upcoming migrations, not the current one.
+         * So we need to save it, and keep it going.
+         */
+        move_charge_at_immigrate  = memcg->move_charge_at_immigrate;
+        if (move_charge_at_immigrate) {
                 struct mm_struct *mm;
                 struct mem_cgroup *from = mem_cgroup_from_task(p);
 
@@ -6580,6 +6585,7 @@ static int mem_cgroup_can_attach(struct cgroup *cgroup,
                         spin_lock(&mc.lock);
                         mc.from = from;
                         mc.to = memcg;
+                        mc.immigrate_flags = move_charge_at_immigrate;
                         spin_unlock(&mc.lock);
                         /* We set mc.moving_task later */