linux-kernel - Re: [PATCH] mm, oom: fix oom_unkillable

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Date:   Mon, 17 Jun 2019 18:17:02 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm, oom: fix oom_unkillable_task for memcg OOMs

On Mon 17-06-19 08:59:54, Shakeel Butt wrote:
> Currently oom_unkillable_task() checks mems_allowed even for memcg OOMs
> which does not make sense as memcg OOMs can not be triggered due to
> numa constraints. Fixing that.
> 
> Also if memcg is given, oom_unkillable_task() will check the task's
> memcg membership as well to detect oom killability. However all the
> memcg related code paths leading to oom_unkillable_task(), other than
> dump_tasks(), come through mem_cgroup_scan_tasks() which traverses
> tasks through memcgs. Once dump_tasks() is converted to use
> mem_cgroup_scan_tasks(), there is no need to do memcg membership check
> in oom_unkillable_task().

I think this patch just does too much in one go. Could you split out
the dump_tasks part and the oom_unkillable_task parts into two patches
please? It should be slightly easier to review.

[...]
> +static bool oom_unkillable_task(struct task_struct *p, struct oom_control *oc)
>  {
>  	if (is_global_init(p))
>  		return true;
>  	if (p->flags & PF_KTHREAD)
>  		return true;
> +	if (!oc)
> +		return false;

Bah, this is just too ugly. AFAICS this is only because oom_score still
uses oom_unkillable_task which is kinda dubious, no? While you are
touching this code, can we remove this part as well? I would be really
surprised if any code really depends on ineligible tasks reporting 0
oom_score.

Other than that it looks reasonable to me from a quick glance but I have
to look more carefuly.
-- 
Michal Hocko
SUSE Labs