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

[Kamezawa Hiroyuki]

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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