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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
References: <9d10df01-0127-fb40-81c3-cc53c9733c3e@virtuozzo.com>
 <YW04jWSv6pQb2Goe@dhcp22.suse.cz>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <6b751abe-aa52-d1d8-2631-ec471975cc3a@virtuozzo.com>
Date:   Mon, 18 Oct 2021 13:05:35 +0300
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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.

We can reach obj_cgroup_charge_pages() for example via

do_user_addr_fault
handle_mm_fault
__handle_mm_fault
p4d_alloc
__p4d_alloc
p4d_alloc_one
get_zeroed_page
__get_free_pages
alloc_pages
__alloc_pages
__memcg_kmem_charge_page
obj_cgroup_charge_pages

Here we call try_charge_memcg() that return success and approve the allocation,
however then we hit into kmem limit and fail the allocation.

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

> 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.

I like this idea, however it may be not enough, at least in scenario described above.

> [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

Thank you, I'll read this discussion.
	Vasily Averin