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Subject: Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited
 task
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Uladzislau Rezki <urezki@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Shakeel Butt <shakeelb@google.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel@openvz.org
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From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <687bf489-f7a7-5604-25c5-0c1a09e0905b@virtuozzo.com>
Date:   Tue, 19 Oct 2021 13:30:06 +0300
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On 19.10.2021 11:49, Michal Hocko wrote:
> On Tue 19-10-21 09:30:18, Vasily Averin wrote:
> [...]
>> With my patch ("memcg: prohibit unconditional exceeding the limit of dying tasks") try_charge_memcg() can fail:
>> a) due to fatal signal
>> b) when mem_cgroup_oom -> mem_cgroup_out_of_memory -> out_of_memory() returns false (when select_bad_process() found nothing)
>>
>> To handle a) we can follow to your suggestion and skip excution of out_of_memory() in pagefault_out_of memory()
>> To handle b) we can go to retry: if mem_cgroup_oom() return OOM_FAILED.

> How is b) possible without current being killed? Do we allow remote
> charging?

out_of_memory for memcg_oom
 select_bad_process
  mem_cgroup_scan_tasks
   oom_evaluate_task
    oom_badness

        /*
         * Do not even consider tasks which are explicitly marked oom
         * unkillable or have been already oom reaped or the are in
         * the middle of vfork
         */
        adj = (long)p->signal->oom_score_adj;
        if (adj == OOM_SCORE_ADJ_MIN ||
                        test_bit(MMF_OOM_SKIP, &p->mm->flags) ||
                        in_vfork(p)) {
                task_unlock(p);
                return LONG_MIN;
        }

This time we handle userspace page fault, so we cannot be kenrel thread,
and cannot be in_vfork().
However task can be marked as oom unkillable, 
i.e. have p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN

It can be set via oom_score_adj_write().

Thank you,
	Vasily Averin