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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 6 Aug 2018 12:39:09 +0200
Message-ID: <CACT4Y+ZEAoPWxEJ2yAf6b5cSjAm+MPx1yrk70BWHRrnDYdyb_A@mail.gmail.com>
Subject: Re: WARNING in try_charge
To:     Michal Hocko <mhocko@kernel.org>
Cc:     syzbot <syzbot+bab151e82a4e973fa325@syzkaller.appspotmail.com>,
        cgroups@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
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On Mon, Aug 6, 2018 at 11:48 AM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 06-08-18 11:30:37, Dmitry Vyukov wrote:
>> On Mon, Aug 6, 2018 at 11:15 AM, Michal Hocko <mhocko@kernel.org> wrote:
> [...]
>> > More interesting stuff is higher in the kernel log
>> > : [  366.435015] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/ile0,task_memcg=/ile0,task=syz-executor3,pid=23766,uid=0
>> > : [  366.449416] memory: usage 112kB, limit 0kB, failcnt 1605
>> >
>> > Are you sure you want to have hard limit set to 0?
>>
>> syzkaller really does not mind to have it.
>
> So what do you use it for? What do you actually test by this setting?
>
> [...]
>> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> > index 4603ad75c9a9..852cd3dbdcd9 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;
>> >
>> > Anyway your memcg setup is indeed misconfigured. Memcg with 0 hard limit
>> > and basically no memory charged by existing tasks is not going to fly
>> > and the warning is exactly to call that out.
>>
>>
>> Please-please-please do not mix kernel bugs and notices to user into
>> the same bucket:
>
> Well, WARN_ON used to be a standard way to make user aware of a
> misbehavior. In this case it warns about a pottential runaway when memcg
> is misconfigured. I do not insist on using WARN_ON here of course. If
> there is a general agreement that such a condition is better handled by
> pr_err then I am fine with it. Users tend to be more sensitive on
> WARN_ONs though.
>
> Btw. running with the above diff on top might help us to ideantify
> whether this is a pre-mature warning or a valid one. Still useful to
> find out.

The bug report has a reproducer, so you can run it with the patch. Or
ask syzbot to test your patch:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches
Which basically boils down to saying:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

Note that a text patch without a base tree/commit can be useless, I
used torvalds/linux.git but I don't know if it will apply there or
not. Let's see.

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