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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: <339ae4b5-6efd-8fc2-33f1-2eb3aee71cb2@virtuozzo.com>
Date:   Tue, 19 Oct 2021 09:30:18 +0300
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On 18.10.2021 14:53, Michal Hocko wrote:
> On Mon 18-10-21 13:05:35, Vasily Averin wrote:
>> On 18.10.2021 12:04, Michal Hocko wrote:
>>> On Mon 18-10-21 11:13:52, Vasily Averin wrote:
>>> [...]
>>>> How could this happen?
>>>>
>>>> User-space task inside the memcg-limited container generated a page fault,
>>>> its handler do_user_addr_fault() called handle_mm_fault which could not
>>>> allocate the page due to exceeding the memcg limit and returned VM_FAULT_OOM.
>>>> Then do_user_addr_fault() called pagefault_out_of_memory() which executed
>>>> out_of_memory() without set of memcg.
>>>>
>>>> Partially this problem depends on one of my recent patches, disabled unlimited
>>>> memory allocation for dying tasks. However I think the problem can happen
>>>> on non-killed tasks too, for example because of kmem limit.
>>>
>>> Could you be more specific on how this can happen without your patch? I
>>> have to say I haven't realized this side effect when discussing it.

>> If required I can try to search how try_charge_memcg() can reject page allocation 
>> of non-dying task too.
> 
> Yes.

Now I think that such failure was very unlikely (w/o my patch and kmem limit).
I cannot exclude it completely, because I did not finished this review and perhaps I missed something,
but I checked most part of code and found nothing.

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.

However all these cases can be successfully handled by my new patch
"memcg: prevent false global OOM triggered by memcg limited task"
and I think it is better solution.

Thank you,
	Vasily Averin