> > 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
