Date: Tue, 15 Dec 2015 14:02:19 +0300
From: Vladimir Davydov <vdavydov@virtuozzo.com>
To: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
CC: Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/7] mm: memcontrol: charge swap to cgroup2
Message-ID: <20151215110219.GJ28521@esperanza>
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 <20151214194258.GH28521@esperanza>
 <566F8781.80108@jp.fujitsu.com>
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On Tue, Dec 15, 2015 at 12:22:41PM +0900, Kamezawa Hiroyuki wrote:
> On 2015/12/15 4:42, Vladimir Davydov wrote:
> >On Mon, Dec 14, 2015 at 04:30:37PM +0100, 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.
> >
> >AFAICS such anon memory protection has a side-effect: real-life
> >workloads need page cache to run smoothly (at least for mapping
> >executables). Disabling swapping would switch pressure to page caches,
> >resulting in performance degradation. So, I don't think per memcg swap
> >limit can be abused to boost your workload on an overcommitted system.
> >
> >If you mean malicious users, well, they already have plenty ways to eat
> >all available memory up to the hard limit by creating unreclaimable
> >kernel objects.
> >
> "protect anon" user's malicious degree is far lower than such cracker like users.

What do you mean by "malicious degree"? What is such a user trying to
achieve? Killing the system? Well, there are much more effective ways to
do so. Or does it want to exploit a system specific feature to get
benefit for itself? If so, it will hardly win by mlocking all anonymous
memory, because this will result in higher pressure exerted upon its
page cache and dcache, which normal workloads just can't get along
without.

> 
> >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.

What do you mean by "easy mistake"? Misconfiguration? If so, it's a lame
excuse IMO. Admin should take system configuration seriously. If the
host is not overcommitted, it's trivial. Otherwise, there's always a
chance that things will go south, so it's not going to be easy. It's up
to admin to analyze risks and set limits accordingly. Exporting knobs
with clear meaning is the best we can do here. swap.max is one such knob
It defines maximal usage of swap resource. Allowing to breach it just
does not add up.

> Can't you add "swap excess" switch to sysctl to allow global memory
> reclaim can ignore swap limitation ?

I'd be opposed to it, because this would obscure the user API. OTOH, a
kind of swap soft limit (swap.high?) might be considered. I'm not sure
if it's really necessary though, because all arguments for it do not
look convincing to me for now. So, personally, I would refrain from
implementing it until it is really called for by users of cgroup v2.

Thanks,
Vladimir
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