linux-kernel - Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2

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Date:	Tue, 15 Dec 2015 18:29:59 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Michal Hocko <mhocko@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2

On 2015/12/15 17:30, Vladimir Davydov wrote:
> On Tue, Dec 15, 2015 at 12:12:40PM +0900, Kamezawa Hiroyuki wrote:
>> On 2015/12/15 0:30, Michal Hocko wrote:
>>> On Thu 10-12-15 14:39:14, Vladimir Davydov wrote:
>>>> In the legacy hierarchy we charge memsw, which is dubious, because:
>>>>
>>>>   - memsw.limit must be >= memory.limit, so it is impossible to limit
>>>>     swap usage less than memory usage. Taking into account the fact that
>>>>     the primary limiting mechanism in the unified hierarchy is
>>>>     memory.high while memory.limit is either left unset or set to a very
>>>>     large value, moving memsw.limit knob to the unified hierarchy would
>>>>     effectively make it impossible to limit swap usage according to the
>>>>     user preference.
>>>>
>>>>   - memsw.usage != memory.usage + swap.usage, because a page occupying
>>>>     both swap entry and a swap cache page is charged only once to memsw
>>>>     counter. As a result, it is possible to effectively eat up to
>>>>     memory.limit of memory pages *and* memsw.limit of swap entries, which
>>>>     looks unexpected.
>>>>
>>>> That said, we should provide a different swap limiting mechanism for
>>>> cgroup2.
>>>> This patch adds mem_cgroup->swap counter, which charges the actual
>>>> number of swap entries used by a cgroup. It is only charged in the
>>>> unified hierarchy, while the legacy hierarchy memsw logic is left
>>>> intact.
>>>
>>> I agree that the previous semantic was awkward. The problem I can see
>>> with this approach is that once the swap limit is reached the anon
>>> memory pressure might spill over to other and unrelated memcgs during
>>> the global memory pressure. I guess this is what Kame referred to as
>>> anon would become mlocked basically. This would be even more of an issue
>>> with resource delegation to sub-hierarchies because nobody will prevent
>>> setting the swap amount to a small value and use that as an anon memory
>>> protection.
>>>
>>> I guess this was the reason why this approach hasn't been chosen before
>>
>> Yes. At that age, "never break global VM" was the policy. And "mlock" can be
>> used for attacking system.
>
> If we are talking about "attacking system" from inside a container,
> there are much easier and disruptive ways, e.g. running a fork-bomb or
> creating pipes - such memory can't be reclaimed and global OOM killer
> won't help.

You're right. We just wanted to avoid affecting global memory reclaim by
each cgroup settings.

Thanks,
-Kame



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