[$] 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 physical
characteristics of the memory pages contained within them. Over time,
zones have taken on more of a policy-related role as well. With a patch
set called THP
allocator optimizations, Yu Zhao has set out to better define the role
of policy-related zones on the path toward adding two more of them, with
the ultimate purpose of improving the kernel's support for transparent huge
pages (THPs).