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

[Pavel Tatashin]

> > 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
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.