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Subject: Re: [PATCH memcg] memcg: prohibit unconditional exceeding the limit
 of dying tasks
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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 <YT8RjxShvfEVe4YU@dhcp22.suse.cz>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <7af26106-388c-6f99-e018-669a8f0cf9b5@virtuozzo.com>
Date:   Mon, 13 Sep 2021 13:35:00 +0300
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On 9/13/21 11:53 AM, Michal Hocko wrote:
> On Fri 10-09-21 15:39:28, Vasily Averin wrote:
>> The kernel currently allows dying tasks to exceed the memcg limits.
>> The allocation is expected to be the last one and the occupied memory
>> will be freed soon.
>> This is not always true because it can be part of the huge vmalloc
>> allocation. Allowed once, they will repeat over and over again.
>> Moreover lifetime of the allocated object can differ from
>> In addition the lifetime of the dying task.
>> Multiple such allocations running concurrently can not only overuse
>> the memcg limit, but can lead to a global out of memory and,
>> in the worst case, cause the host to panic.
>>
>> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
>> ---
>>  mm/memcontrol.c | 23 +++++------------------
>>  1 file changed, 5 insertions(+), 18 deletions(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 389b5766e74f..67195fcfbddf 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -1834,6 +1834,9 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
>>  		return OOM_ASYNC;
>>  	}
>>  
>> +	if (should_force_charge())
>> +		return OOM_SKIPPED;
> 
> mem_cgroup_out_of_memory already check for the bypass, now you are
> duplicating that check with a different answer to the caller. This is
> really messy. One of the two has to go away.

In this case mem_cgroup_out_of_memory() takes locks and mutexes but doing nothing
useful and its success causes try_charge_memcg() to repeat the loop unnecessarily.

I cannot change mem_cgroup_out_of_memory internals, because it is used in other places too.The check inside mem_cgroup_out_of_memory is required because situation can be changed after
check added into mem_cgroup_oom().

Though I got your argument, and will think how to improve the patch.
Anyway we'll need to do something with name of should_force_charge() function
that will NOT lead to forced charge.

Thank you,
	Vasily Averin

Thank you,