Re: [PATCH v2] oom, oom_reaper: do not enqueue same task twice

[Michal Hocko]

On Sat 26-01-19 22:10:52, Tetsuo Handa wrote:
[...]
> >From 9c9e935fc038342c48461aabca666f1b544e32b1 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Sat, 26 Jan 2019 21:57:25 +0900
> Subject: [PATCH v2] oom, oom_reaper: do not enqueue same task twice
> 
> Arkadiusz reported that enabling memcg's group oom killing causes
> strange memcg statistics where there is no task in a memcg despite
> the number of tasks in that memcg is not 0. It turned out that there
> is a bug in wake_oom_reaper() which allows enqueuing same task twice
> which makes impossible to decrease the number of tasks in that memcg
> due to a refcount leak.
> 
> This bug existed since the OOM reaper became invokable from
> task_will_free_mem(current) path in out_of_memory() in Linux 4.7,
> but memcg's group oom killing made it easier to trigger this bug by
> calling wake_oom_reaper() on the same task from one out_of_memory()
> request.
> 
> Fix this bug using an approach used by commit 855b018325737f76
> ("oom, oom_reaper: disable oom_reaper for oom_kill_allocating_task").
> As a side effect of this patch, this patch also avoids enqueuing
> multiple threads sharing memory via task_will_free_mem(current) path.

Thanks for the analysis and the patch. This should work, I believe but
I am not really thrilled to overload the meaning of the MMF_UNSTABLE.
The flag is meant to signal accessing address space is not stable and it
is not aimed to synchronize oom reaper with the oom path.

Can we make use mark_oom_victim directly? I didn't get to think that
through right now so I might be missing something but this should
prevent repeating queueing as well.

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f0e8cd9edb1a..dac4f2197e53 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -690,7 +690,7 @@ static void mark_oom_victim(struct task_struct *tsk)
 	WARN_ON(oom_killer_disabled);
 	/* OOM killer might race with memcg OOM */
 	if (test_and_set_tsk_thread_flag(tsk, TIF_MEMDIE))
-		return;
+		return false;
 
 	/* oom_mm is bound to the signal struct life time. */
 	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {
@@ -707,6 +707,8 @@ static void mark_oom_victim(struct task_struct *tsk)
 	__thaw_task(tsk);
 	atomic_inc(&oom_victims);
 	trace_mark_victim(tsk->pid);
+
+	return true;
 }
 
 /**
@@ -873,7 +875,7 @@ static void __oom_kill_process(struct task_struct *victim)
 	 * reserves from the user space under its control.
 	 */
 	do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
-	mark_oom_victim(victim);
+	can_oom_reap = mark_oom_victim(victim);
 	pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
 		task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
 		K(get_mm_counter(victim->mm, MM_ANONPAGES)),
@@ -954,8 +956,8 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	 */
 	task_lock(p);
 	if (task_will_free_mem(p)) {
-		mark_oom_victim(p);
-		wake_oom_reaper(p);
+		if (mark_oom_victim(p)
+			wake_oom_reaper(p);
 		task_unlock(p);
 		put_task_struct(p);
 		return;
@@ -1084,8 +1086,8 @@ bool out_of_memory(struct oom_control *oc)
 	 * quickly exit and free its memory.
 	 */
 	if (task_will_free_mem(current)) {
-		mark_oom_victim(current);
-		wake_oom_reaper(current);
+		if (mark_oom_victim(current))
+			wake_oom_reaper(current);
 		return true;
 	}
 
-- 
Michal Hocko
SUSE Labs