Subject: Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2
To: Johannes Weiner <hannes@cmpxchg.org>
References: <cover.1449742560.git.vdavydov@virtuozzo.com>
 <265d8fe623ed2773d69a26d302eb31e335377c77.1449742560.git.vdavydov@virtuozzo.com>
 <20151214153037.GB4339@dhcp22.suse.cz> <20151214194258.GH28521@esperanza>
 <566F8781.80108@jp.fujitsu.com> <20151215145011.GA20355@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
From: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Message-ID: <5670D806.60408@jp.fujitsu.com>
Date: Wed, 16 Dec 2015 12:18:30 +0900
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On 2015/12/15 23:50, Johannes Weiner wrote:
> On Tue, Dec 15, 2015 at 12:22:41PM +0900, Kamezawa Hiroyuki wrote:
>> On 2015/12/15 4:42, Vladimir Davydov wrote:
>>> Anyway, if you don't trust a container you'd better set the hard memory
>>> limit so that it can't hurt others no matter what it runs and how it
>>> tweaks its sub-tree knobs.
>>
>> Limiting swap can easily cause "OOM-Killer even while there are available swap"
>> with easy mistake. Can't you add "swap excess" switch to sysctl to allow global
>> memory reclaim can ignore swap limitation ?
>
> That never worked with a combined memory+swap limit, either. How could
> it? The parent might swap you out under pressure, but simply touching
> a few of your anon pages causes them to get swapped back in, thrashing
> with whatever the parent was trying to do. Your ability to swap it out
> is simply no protection against a group touching its pages.
>
> Allowing the parent to exceed swap with separate counters makes even
> less sense, because every page swapped out frees up a page of memory
> that the child can reuse. For every swap page that exceeds the limit,
> the child gets a free memory page! The child doesn't even have to
> cause swapin, it can just steal whatever the parent tried to free up,
> and meanwhile its combined memory & swap footprint explodes.
>
Sure.

> The answer is and always should have been: don't overcommit untrusted
> cgroups. Think of swap as a resource you distribute, not as breathing
> room for the parents to rely on. Because it can't and could never.
>
ok, don't overcommmit.

> And the new separate swap counter makes this explicit.
>
Hmm, my requests are
  - set the same capabilities as mlock() to set swap.limit=0
  - swap-full notification via vmpressure or something mechanism.
  - OOM-Killer's available memory calculation may be corrupted, please check.
  - force swap-in at reducing swap.limit

Thanks,
-Kame


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