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Date:   Mon, 6 Aug 2018 20:13:39 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     syzbot <syzbot+bab151e82a4e973fa325@syzkaller.appspotmail.com>
Cc:     cgroups@vger.kernel.org, dvyukov@google.com, hannes@cmpxchg.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        penguin-kernel@I-love.SAKURA.ne.jp,
        syzkaller-bugs@googlegroups.com, vdavydov.dev@gmail.com
Subject: Re: WARNING in try_charge
Message-ID: <20180806181339.GD10003@dhcp22.suse.cz>
References: <fc6e173e-8bda-269f-d44f-1c5f5215beac@I-love.SAKURA.ne.jp>
 <0000000000006350880572c61e62@google.com>
 <20180806174410.GB10003@dhcp22.suse.cz>
 <20180806175627.GC10003@dhcp22.suse.cz>
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I simply do not see how this is possible. Let's try with the following
extended debugging patch.

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git 116b181bb646afedd770985de20a68721bdb2648

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4603ad75c9a9..e2dfdf4361ba 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1388,6 +1388,8 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	bool ret;
 
 	mutex_lock(&oom_lock);
+	pr_info("task=%s pid=%d invoked memcg oom killer. oom_victim=%d\n",
+			current->comm, current->pid, tsk_is_oom_victim(current));
 	ret = out_of_memory(&oc);
 	mutex_unlock(&oom_lock);
 	return ret;
@@ -2108,6 +2110,9 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 
 	if (mem_cgroup_is_root(memcg))
 		return 0;
+	if (tsk_is_oom_victim(current))
+		pr_info("task=%s pid=%d charge for nr_pages=%d\n",
+			current->comm, current->pid, nr_pages);
 retry:
 	if (consume_stock(memcg, nr_pages))
 		return 0;
@@ -2137,8 +2142,11 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	 */
 	if (unlikely(tsk_is_oom_victim(current) ||
 		     fatal_signal_pending(current) ||
-		     current->flags & PF_EXITING))
+		     current->flags & PF_EXITING)) {
+		pr_info("task=%s pid=%d charge bypass\n",
+			current->comm, current->pid);
 		goto force;
+	}
 
 	/*
 	 * Prevent unbounded recursion when reclaim operations need to
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 104ef4a01a55..7d9adcde8cf6 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -692,6 +692,8 @@ static void mark_oom_victim(struct task_struct *tsk)
 	__thaw_task(tsk);
 	atomic_inc(&oom_victims);
 	trace_mark_victim(tsk->pid);
+	pr_info("task=%s pid=%d is oom victim now\n",
+			current->comm, current->pid);
 }
 
 /**
-- 
Michal Hocko
SUSE Labs