[PATCH v2 1/7] memcg: prevent changes to move_charge_at_immigrate during task attach

[Glauber Costa <glommer@xxxxxxxxxxxxx>]

Currently, we rely on the cgroup_lock() to prevent changes to
move_charge_at_immigrate during task migration. However, this is only
needed because the current strategy keeps checking this value throughout
the whole process. Since all we need is serialization, one needs only to
guarantee that whatever decision we made in the beginning of a specific
migration is respected throughout the process.

We can achieve this by just saving it in mc. By doing this, no kind of
locking is needed.

Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
---
 mm/memcontrol.c | 26 +++++++++++++++++---------
 1 file changed, 17 insertions(+), 9 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 09255ec..18f4e76 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -412,6 +412,7 @@ static struct move_charge_struct {
 	spinlock_t	  lock; /* for from, to */
 	struct mem_cgroup *from;
 	struct mem_cgroup *to;
+	unsigned long move_charge_at_immigrate;
 	unsigned long precharge;
 	unsigned long moved_charge;
 	unsigned long moved_swap;
@@ -425,13 +426,13 @@ static struct move_charge_struct {
 static bool move_anon(void)
 {
 	return test_bit(MOVE_CHARGE_TYPE_ANON,
-					&mc.to->move_charge_at_immigrate);
+					&mc.move_charge_at_immigrate);
 }
 
 static bool move_file(void)
 {
 	return test_bit(MOVE_CHARGE_TYPE_FILE,
-					&mc.to->move_charge_at_immigrate);
+					&mc.move_charge_at_immigrate);
 }
 
 /*
@@ -5146,15 +5147,14 @@ 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
@@ -6530,8 +6530,15 @@ 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);
 
@@ -6551,6 +6558,7 @@ static int mem_cgroup_can_attach(struct cgroup *cgroup,
 			spin_lock(&mc.lock);
 			mc.from = from;
 			mc.to = memcg;
+			mc.move_charge_at_immigrate = move_charge_at_immigrate;
 			spin_unlock(&mc.lock);
 			/* We set mc.moving_task later */
 
-- 
1.7.11.7

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