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From: Dmitry Vyukov <dvyukov@google.com>
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>,
	Dmitry Torokhov <dtor@google.com>
Subject: Re: WARNING in try_charge
Date: Mon, 6 Aug 2018 17:07:26 +0200
Message-ID: <CACT4Y+bbKG4SQoEY+DVYRzyYN4aBJ1goC+B1R26dGtMVPkMonQ@mail.gmail.com> (raw)
In-Reply-To: <20180806142124.GP19540@dhcp22.suse.cz>

On Mon, Aug 6, 2018 at 4:21 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 06-08-18 13:57:38, Dmitry Vyukov wrote:
>> On Mon, Aug 6, 2018 at 1:02 PM, Michal Hocko <mhocko@kernel.org> wrote:
> [...]
>> >> A much
>> >> friendlier for user way to say this would be print a message at the
>> >> point of misconfiguration saying what exactly is wrong, e.g. "pid $PID
>> >> misconfigures cgroup /cgroup/path with mem.limit=0" without a stack
>> >> trace (does not give any useful info for user). And return EINVAL if
>> >> it can't fly at all? And then leave the "or a kernel bug" part for the
>> >> WARNING each occurrence of which we do want to be reported to kernel
>> >> developers.
>> >
>> > But this is not applicable here. Your misconfiguration is quite obvious
>> > because you simply set the hard limit to 0. This is not the only
>> > situation when this can happen. There is no clear point to tell, you are
>> > doing this wrong. If it was we would do it at that point obviously.
>>
>> But, isn't there a point were hard limit is set to 0? I would expect
>> there is a something like cgroup file write handler with a value of 0
>> or something.
>
> Yeah, but this is only one instance of the problem. Other is that the
> memcg is not reclaimable for any other reasons. And we do not know what
> those might be
>
>>
>> > If you have a strong reason to believe that this is an abuse of WARN I
>> > am all happy to change that. But I haven't heard any yet, to be honest.
>>
>> WARN must not be used for anything that is not kernel bugs. If this is
>> not kernel bug, WARN must not be used here.
>
> This is rather strong wording without any backing arguments. I strongly
> doubt 90% of existing WARN* match this expectation. WARN* has
> traditionally been a way to tell that something suspicious is going on.
> Those situation are mostly likely not fatal but it is good to know they
> are happening.

Today syzbot covers about 1M lines of kernel code, and we fuzz for
several years with panic_on_warn=1 and each unique crash is recorded
and reported. Over several thousands bugs that we reported, there were
maybe 2 dozens of such cases (WARN on invalid user inputs, ENOMEM,
etc). The solution always was to remove the WARNING on covert to
pr_err. As of now, I see only 2 such cases open: this one and WARN on
ENOMEM in input subsystem.

Either way, we do badly need this separation. If there are deviations
we need to continue fixing them.

  parent reply index

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-04 13:33 syzbot
2018-08-04 13:45 ` Tetsuo Handa
2018-08-05 11:33   ` Tetsuo Handa
2018-08-05  8:14 ` syzbot
2018-08-06  9:15 ` Michal Hocko
2018-08-06  9:30   ` Dmitry Vyukov
2018-08-06  9:48     ` Michal Hocko
2018-08-06 10:34       ` Dmitry Vyukov
2018-08-06 11:02         ` Michal Hocko
2018-08-06 11:57           ` Dmitry Vyukov
2018-08-06 14:21             ` Michal Hocko
2018-08-06 14:58               ` Dmitry Vyukov
2018-08-06 17:30                 ` Michal Hocko
2018-08-06 17:53                   ` Dmitry Vyukov
2018-08-06 15:07               ` Dmitry Vyukov [this message]
2018-08-06 15:31               ` Johannes Weiner
2018-08-06 10:39       ` Dmitry Vyukov
2018-08-06 10:47         ` Tetsuo Handa
2018-08-06 11:09           ` Michal Hocko
2018-08-06 11:27           ` syzbot
2018-08-06 11:32             ` Michal Hocko
2018-08-06 11:58               ` Dmitry Vyukov
2018-08-06 14:41               ` Tetsuo Handa
2018-08-06 14:58                 ` Michal Hocko
2018-08-06 15:12                   ` Tetsuo Handa
2018-08-06 14:54               ` David Howells
2018-08-06 15:04                 ` Tetsuo Handa
2018-08-06 11:00         ` syzbot
2018-08-06 15:32         ` Tetsuo Handa
2018-08-06 15:42           ` syzbot
2018-08-06 16:02             ` Tetsuo Handa
2018-08-06 17:44             ` Michal Hocko
2018-08-06 17:49               ` Dmitry Vyukov
2018-08-06 17:56               ` Michal Hocko
2018-08-06 18:13                 ` Michal Hocko
2018-08-06 18:23                   ` syzbot
2018-08-06 18:55                     ` Michal Hocko
2018-08-06 19:12                       ` syzbot
2018-08-06 19:45                         ` Michal Hocko
2018-08-06 19:46                           ` Michal Hocko
2018-08-07 11:18                       ` Dmitry Vyukov
2018-08-07 11:25                         ` Michal Hocko
2018-08-06 18:39                   ` Michal Hocko
2018-08-06 20:26                 ` Tetsuo Handa
2018-08-06 20:34                   ` Michal Hocko
2018-08-06 20:46                     ` Tetsuo Handa
2018-08-06 20:55                       ` Michal Hocko
2018-08-06 21:50                         ` Tetsuo Handa
2018-08-07 10:19                           ` Tetsuo Handa
2018-08-09 13:57 ` Tetsuo Handa
2018-08-09 15:07   ` Michal Hocko
2018-08-09 21:05     ` Tetsuo Handa
2018-08-09 15:34   ` Johannes Weiner

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