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

[Pavel Tatashin <pasha.tatashin@soleen.com>]

> > 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
> > functionality, and not availability of THP, however, I did not know
> > about the use case where some admins might configure ZONE_MOVABLE to
> > increase availability of THP because pages are always migratable in
> > them. The thing is, if we fragment ZONE_MOVABLE by pinning pages in
> > it, the availability of THP also suffers.  We can migrate pages in
> > ZONE_NORMAL, just not guaranteed, so we can create THP in ZONE_NORMAL
> > as well, which is the usual case.
>
> Right, we should document this at some place to make admins aware of
> this. Something like
>
> "Techniques that rely on long-term pinnings of memory (especially, RDMA
> and vfio) are fundamentally problematic with ZONE_MOVABLE and,
> therefore, memory hotunplug. Pinned pages cannot reside on ZONE_MOVABLE,
> to guarantee that memory can still get hotunplugged - be aware that
> pinning can fail even if there is plenty of free memory in ZONE_MOVABLE.
> In addition, using ZONE_MOVABLE might make page pinning more expensive,
> because pages have to be migrated off that zone first."

Thanks, I will add this.

>
> BTW, you might also want to update the comment for ZONE_MOVABLE in
> include/linux/mmzone.h at the end of this series, removing the special
> case of pinned pages (1.) and maybe adding what happens when trying to
> pin pages on ZONE_MOVABLE.

Will do it.

Thank you,
Pasha