staging: android/lowmemorykiller: Better mm handling

LMK should not directly check for task->mm. The reason is that the
process' threads may exit or detach its mm via use_mm(), but other
threads may still have a valid mm. To catch this we use
find_lock_task_mm(), which walks up all threads and returns an
appropriate task (with lock held).
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/lowmemorykiller.c

@@ -82,7 +82,7 @@ task_notify_func(struct notifier_block *self, unsigned long val, void *data)
 
 static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 {
-	struct task_struct *p;
+	struct task_struct *tsk;
 	struct task_struct *selected = NULL;
 	int rem = 0;
 	int tasksize;
@@ -134,15 +134,17 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 	selected_oom_adj = min_adj;
 
 	rcu_read_lock();
-	for_each_process(p) {
-		struct mm_struct *mm;
+	for_each_process(tsk) {
+		struct task_struct *p;
 		struct signal_struct *sig;
 		int oom_adj;
 
-		task_lock(p);
-		mm = p->mm;
+		p = find_lock_task_mm(tsk);
+		if (!p)
+			continue;
+
 		sig = p->signal;
-		if (!mm || !sig) {
+		if (!sig) {
 			task_unlock(p);
 			continue;
 		}
@@ -151,7 +153,7 @@ static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 			task_unlock(p);
 			continue;
 		}
-		tasksize = get_mm_rss(mm);
+		tasksize = get_mm_rss(p->mm);
 		task_unlock(p);
 		if (tasksize <= 0)
 			continue;