linux-kernel - Re: [PATCH] memcg, oom: move out_of

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Date:   Tue, 03 Jul 2018 00:08:05 -0700
From:   Greg Thelen <gthelen@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg, oom: move out_of_memory back to the charge path

Michal Hocko <mhocko@...nel.org> wrote:

> On Fri 29-06-18 11:59:04, Greg Thelen wrote:
>> Michal Hocko <mhocko@...nel.org> wrote:
>> 
>> > On Thu 28-06-18 16:19:07, Greg Thelen wrote:
>> >> Michal Hocko <mhocko@...nel.org> wrote:
>> > [...]
>> >> > +	if (mem_cgroup_out_of_memory(memcg, mask, order))
>> >> > +		return OOM_SUCCESS;
>> >> > +
>> >> > +	WARN(1,"Memory cgroup charge failed because of no reclaimable memory! "
>> >> > +		"This looks like a misconfiguration or a kernel bug.");
>> >> 
>> >> I'm not sure here if the warning should here or so strongly worded.  It
>> >> seems like the current task could be oom reaped with MMF_OOM_SKIP and
>> >> thus mem_cgroup_out_of_memory() will return false.  So there's nothing
>> >> alarming in that case.
>> >
>> > If the task is reaped then its charges should be released as well and
>> > that means that we should get below the limit. Sure there is some room
>> > for races but this should be still unlikely. Maybe I am just
>> > underestimating though.
>> >
>> > What would you suggest instead?
>> 
>> I suggest checking MMF_OOM_SKIP or deleting the warning.
>
> So what do you do when you have MMF_OOM_SKIP task? Do not warn? Checking
> for all the tasks would be quite expensive and remembering that from the
> task selection not nice either. Why do you think it would help much?

I assume we could just check current's MMF_OOM_SKIP - no need to check
all tasks.  My only (minor) objection is that the warning text suggests
misconfiguration or kernel bug, when there may be neither.

> I feel strongly that we have to warn when bypassing the charge limit
> during the corner case because it can lead to unexpected behavior and
> users should be aware of this fact. I am open to the wording or some
> optimizations. I would prefer the latter on top with a clear description
> how it helped in a particular case though. I would rather not over
> optimize now without any story to back it.

I'm fine with the warning.  I know enough to look at dmesg logs to take
an educates that the race occurred.  We can refine it later if/when the
reports start rolling in.  No change needed.