Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE

[Michal Hocko]

On Mon 07-12-20 16:12:50, Joonsoo Kim wrote:
> On Fri, Dec 04, 2020 at 12:50:56PM -0500, Pavel Tatashin wrote:
> > > > Yes, this indeed could be a problem for some configurations. I will
> > > > add your comment to the commit log of one of the patches.
> > >
> > > It sounds like there is some inherent tension here, breaking THP's
> > > when doing pin_user_pages() is a really nasty thing to do. DMA
> > > benefits greatly from THP.
> > >
> > > I know nothing about ZONE_MOVABLE, is this auto-setup or an admin
> > > option? If the result of this patch is standard systems can no longer
> > > pin > 80% of their memory I have some regression concerns..
> > 
> > ZONE_MOVABLE can be configured via kernel parameter, or when memory
> > nodes are onlined after hot-add; so this is something that admins
> > configure. ZONE_MOVABLE is designed to gurantee memory hot-plug
> 
> Just note, the origin of ZONE_MOVABLE is to provide availability of
> huge page, especially, hugetlb page. AFAIK, not guarantee memory
> hot-plug. See following commit that introduces the ZONE_MOVABLE.
> 
> 2a1e274 Create the ZONE_MOVABLE zone
> 
> > functionality, and not availability of THP, however, I did not know
> > about the use case where some admins might configure ZONE_MOVABLE to
> 
> The usecase is lightly mentioned in previous discussion.
> 
> http://lkml.kernel.org/r/alpine.DEB.2.23.453.2011221300100.2830030@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> Anyway, I agree with your other arguments and this patchset.

Yes, historically the original motivation for the movable zone was to
help creating large pages via compaction. I also do remember Mel
not being particularly happy about that.

The thing is that the movability constrain is just too strict for this
usecases because the movable zone, especially a lot of it, might be
causing similar to lowmem/highmem problems very well known from 32b
world. So an admin had to be always very careful when configuring to not
cause zone pressure problems.

Later on, with a higher demand on the memory hotplug - especially the
hotremove usecases - it has become clear that the only reliable way for
the memory offlining is to rule out any unmovable memory out of the way
and that is why a rather strong properly of movable zone was relied on.

In the end we are in two rather different requirements here. One for
optimization and one for correctness. In this case I would much rather
focus on the correctness aspect.
-- 
Michal Hocko
SUSE Labs