linux-kernel - Re: WARNING in try

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Message-ID: <20180806142124.GP19540@dhcp22.suse.cz>
Date:   Mon, 6 Aug 2018 16:21:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+bab151e82a4e973fa325@...kaller.appspotmail.com>,
        cgroups@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Dmitry Torokhov <dtor@...gle.com>
Subject: Re: WARNING in try_charge

On Mon 06-08-18 13:57:38, Dmitry Vyukov wrote:
> On Mon, Aug 6, 2018 at 1:02 PM, Michal Hocko <mhocko@...nel.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.

Sure there is that panic_on_warn thingy which you seem to be using and I
suspect it is a reason why you are so careful about warnings in general
but my experience tells me that this configuration is barely usable
except for testing (which is your case).

But as I've said, I do not insist on WARN here. All I care about is to
warn user that something might go south and this may be either due to
misconfiguration or a subtly wrong memcg reclaim/OOM handler behavior.
-- 
Michal Hocko
SUSE Labs