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

[Kamezawa Hiroyuki]

On 2015/12/15 20:02, Vladimir Davydov wrote:
> 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.

I wanted to say almost all application developers want to set swap.limit=0 if allowed.
So, it's a usual people who can kill the system if swap imbalance is allowed.
 
> > > 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.

Considering my customers, running OOM-Killer while there are free swap space is
system's error rather than their misconfiguration.

BTW, mlock() requires CAP_IPC_LOCK.
please set default unlimited and check capability at setting swap limit, at least.

Thanks,
-Kame


> Thanks,
> Vladimir
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