linux-kernel - Re: [PATCH memcg 3/3] memcg: handle memcg oom failures

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Date:   Wed, 20 Oct 2021 15:02:19 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH memcg 3/3] memcg: handle memcg oom failures

On Wed 20-10-21 15:14:27, Vasily Averin wrote:
> mem_cgroup_oom() can fail if current task was marked unkillable
> and oom killer cannot find any victim.
> 
> Currently we force memcg charge for such allocations,
> however it allow memcg-limited userspace task in to overuse assigned limits
> and potentially trigger the global memory shortage.

You should really go into more details whether that is a practical
problem to handle. OOM_FAILED means that the memcg oom killer couldn't
find any oom victim so it cannot help with a forward progress. There are
not that many situations when that can happen. Naming that would be
really useful.
 
> Let's fail the memory charge in such cases.
> 
> This failure should be somehow recognised in #PF context,

explain why

> so let's use current->memcg_in_oom == (struct mem_cgroup *)OOM_FAILED
> 
> ToDo: what is the best way to notify pagefault_out_of_memory() about 
>     mem_cgroup_out_of_memory failure ?

why don't you simply remove out_of_memory from pagefault_out_of_memory
and leave it only with the blocking memcg OOM handling? Wouldn't that be a
more generic solution? Your first patch already goes that way partially.

This change is more risky than the first one. If somebody returns
VM_FAULT_OOM without invoking allocator then it can loop for ever but
invoking OOM killer in such a situation is equally wrong as the oom
killer cannot really help, right?
-- 
Michal Hocko
SUSE Labs