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

[Kamezawa Hiroyuki]

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.

> 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 ?

Regards,
-Kame








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