android, lowmemorykiller: remove task handoff notifier

The task handoff notifier leaks task_struct since it never gets freed
after the callback returns NOTIFY_OK, which means it is responsible for
doing so.

It turns out the lowmemorykiller actually doesn't need this notifier at
all. It's used to prevent unnecessary killing by waiting for a thread
to exit as a result of lowmem_shrink(), however, it's possible to do
this in the same way the kernel oom killer works by setting TIF_MEMDIE
and avoid killing if we're still waiting for it to exit.

The kernel oom killer will already automatically set TIF_MEMDIE for
threads that are attempting to allocate memory that have a fatal signal.
The thread selected by lowmem_shrink() will have such a signal after the
lowmemorykiller sends it a SIGKILL, so this won't result in an
unnecessary use of memory reserves for the thread to exit.

This has the added benefit that we don't have to rely on
CONFIG_PROFILING to prevent needlessly killing tasks.

Reported-by: Werner Landgraf <[e-mail]>
Cc: [e-mail]
Signed-off-by: David Rientjes <[e-mail]>
Acked-by: Colin Cross <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/staging/android/lowmemorykiller.c

···
 };
 static int lowmem_minfree_size = 4;
 
-static struct task_struct *lowmem_deathpending;
 static unsigned long lowmem_deathpending_timeout;
 
 #define lowmem_print(level, x...)			\
···
 		if (lowmem_debug_level >= (level))	\
 			printk(x);			\
 	} while (0)
-
-static int
-task_notify_func(struct notifier_block *self, unsigned long val, void *data);
-
-static struct notifier_block task_nb = {
-	.notifier_call	= task_notify_func,
-};
-
-static int
-task_notify_func(struct notifier_block *self, unsigned long val, void *data)
-{
-	struct task_struct *task = data;
-
-	if (task == lowmem_deathpending)
-		lowmem_deathpending = NULL;
-
-	return NOTIFY_OK;
-}
 
 static int lowmem_shrink(struct shrinker *s, struct shrink_control *sc)
 {
···
 	int other_free = global_page_state(NR_FREE_PAGES);
 	int other_file = global_page_state(NR_FILE_PAGES) -
 						global_page_state(NR_SHMEM);
-
-	/*
-	 * If we already have a death outstanding, then
-	 * bail out right away; indicating to vmscan
-	 * that we have nothing further to offer on
-	 * this pass.
-	 *
-	 * Note: Currently you need CONFIG_PROFILING
-	 * for this to work correctly.
-	 */
-	if (lowmem_deathpending &&
-	    time_before_eq(jiffies, lowmem_deathpending_timeout))
-		return 0;
 
 	if (lowmem_adj_size < array_size)
 		array_size = lowmem_adj_size;
···
 		if (!p)
 			continue;
 
+		if (test_tsk_thread_flag(p, TIF_MEMDIE) &&
+		    time_before_eq(jiffies, lowmem_deathpending_timeout)) {
+			task_unlock(p);
+			rcu_read_unlock();
+			return 0;
+		}
 		oom_score_adj = p->signal->oom_score_adj;
 		if (oom_score_adj < min_score_adj) {
 			task_unlock(p);
···
 		lowmem_print(1, "send sigkill to %d (%s), adj %d, size %d\n",
 			     selected->pid, selected->comm,
 			     selected_oom_score_adj, selected_tasksize);
-		/*
-		 * If CONFIG_PROFILING is off, then we don't want to stall
-		 * the killer by setting lowmem_deathpending.
-		 */
-#ifdef CONFIG_PROFILING
-		lowmem_deathpending = selected;
 		lowmem_deathpending_timeout = jiffies + HZ;
-#endif
 		send_sig(SIGKILL, selected, 0);
+		set_tsk_thread_flag(selected, TIF_MEMDIE);
 		rem -= selected_tasksize;
 	}
 	lowmem_print(4, "lowmem_shrink %lu, %x, return %d\n",
···
 
 static int __init lowmem_init(void)
 {
-	task_handoff_register(&task_nb);
 	register_shrinker(&lowmem_shrinker);
 	return 0;
 }
···
 static void __exit lowmem_exit(void)
 {
 	unregister_shrinker(&lowmem_shrinker);
-	task_handoff_unregister(&task_nb);
 }
 
 module_param_named(cost, lowmem_shrinker.seeks, int, S_IRUGO | S_IWUSR);