[PATCH 8/8] memcg: avoid oom during charge migration

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

This charge migration feature has double charges on both "from" and "to" mem_cgroup
during charge migration.
This means unnecessary oom can happen because of charge migration.

This patch tries to avoid such oom.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c8542e7..73da7e7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -288,6 +288,8 @@ struct migrate_charge {
 	struct list_head list;
 };
 static struct migrate_charge *mc;
+static struct task_struct *mc_task;
+static DECLARE_WAIT_QUEUE_HEAD(mc_waitq);
 
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
@@ -1318,6 +1320,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	while (1) {
 		int ret = 0;
 		unsigned long flags = 0;
+		DEFINE_WAIT(wait);
 
 		if (mem_cgroup_is_root(mem))
 			goto done;
@@ -1359,6 +1362,17 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 		if (mem_cgroup_check_under_limit(mem_over_limit))
 			continue;
 
+		/* try to avoid oom while someone is migrating charge */
+		if (current != mc_task) {
+			prepare_to_wait(&mc_waitq, &wait, TASK_INTERRUPTIBLE);
+			if (mc) {
+				schedule();
+				finish_wait(&mc_waitq, &wait);
+				continue;
+			}
+			finish_wait(&mc_waitq, &wait);
+		}
+
 		if (!nr_retries--) {
 			if (oom) {
 				mutex_lock(&memcg_tasklist);
@@ -3432,6 +3446,8 @@ static void mem_cgroup_clear_migrate_charge(void)
 
 	kfree(mc);
 	mc = NULL;
+	mc_task = NULL;
+	wake_up_all(&mc_waitq);
 }
 
 static int mem_cgroup_can_migrate_charge(struct mem_cgroup *mem,
@@ -3441,6 +3457,7 @@ static int mem_cgroup_can_migrate_charge(struct mem_cgroup *mem,
 	struct mem_cgroup *from = mem_cgroup_from_task(p);
 
 	VM_BUG_ON(mc);
+	VM_BUG_ON(mc_task);
 
 	if (from == mem)
 		return 0;
@@ -3453,6 +3470,7 @@ static int mem_cgroup_can_migrate_charge(struct mem_cgroup *mem,
 	mc->from = from;
 	mc->to = mem;
 	INIT_LIST_HEAD(&mc->list);
+	mc_task = current;
 
 	ret = migrate_charge_prepare();
 

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