[PATCH][mmotm]memcg: handle null dereference of mm->owner

[Daisuke Nishimura]

Hi.

mm_update_next_owner() may clear mm->owner to NULL
if it races with swapoff, page migration, etc.
(This behavior was introduced by mm-owner-fix-race-between-swap-and-exit.patch.)

But memcg doesn't take account of this situation, and causes:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000630

This fixes it.


Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

---
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2979d22..ec2c16b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -244,6 +244,14 @@ static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)
 
 struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
 {
+	/*
+	 * mm_update_next_owner() may clear mm->owner to NULL
+	 * if it races with swapoff, page migration, etc.
+	 * So this can be called with p == NULL.
+	 */
+	if (unlikely(!p))
+		return NULL;
+
 	return container_of(task_subsys_state(p, mem_cgroup_subsys_id),
 				struct mem_cgroup, css);
 }
@@ -534,6 +542,11 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm,
 	if (likely(!memcg)) {
 		rcu_read_lock();
 		mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
+		if (unlikely(!mem)) {
+			rcu_read_unlock();
+			kmem_cache_free(page_cgroup_cache, pc);
+			return 0;
+		}
 		/*
 		 * For every charge from the cgroup, increment reference count
 		 */
@@ -790,6 +803,10 @@ int mem_cgroup_shrink_usage(struct mm_struct *mm, gfp_t gfp_mask)
 
 	rcu_read_lock();
 	mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
+	if (unlikely(!mem)) {
+		rcu_read_unlock();
+		return 0;
+	}
 	css_get(&mem->css);
 	rcu_read_unlock();
 
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