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Date:   Mon, 18 Oct 2021 13:53:43 +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 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.
> 
> 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.

Just to make sure I understand this would be for the v1 kmem explicit
limit, correct?

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

Yes.

> > 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.

I original patch has removed the oom killer completely.

-- 
Michal Hocko
SUSE Labs