[$] Formalizing policy zones for memory

The kernel's memory-management subsystem is built on the concept of"zones", which were initially added to describe the physicalcharacteristics of the memory pages contained within them. Over time,zones have taken on more of a policy-related role as well. With a patchset called THPallocator optimizations, Yu Zhao has set out to better define the roleof policy-related zones on the path toward adding two more of them, withthe ultimate purpose of improving the kernel's support for transparent hugepages (THPs).