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Date:   Mon, 18 Oct 2021 11:04:13 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Vasily Averin <vvs@virtuozzo.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
Subject: Re: [PATCH memcg 0/1] false global OOM triggered by memcg-limited
 task
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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.

I will be honest that I am not really happy about pagefault_out_of_memory.
I have tried to remove it in the past. Without much success back then,
unfortunately[1]. 
Maybe we should get rid of it finally. The OOM is always triggered from
inside the allocator where we have much more infromation about the
allocation context. A first step would be to skip pagefault_out_of_memory
for killed or exiting processes.

[1] I do not have msg-id so I cannot provide a lore link but google
pointed me to https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1400402.html
-- 
Michal Hocko
SUSE Labs