[$] Hot-page migration and specific-purpose NUMA nodes

For better or for worse, the NUMA node is the abstraction used by the
kernel to keep track of different types of memory. How that abstraction is
used, though, is still an active area of development. Two patch sets
focused on this problem are currently under review; one addresses the
perennial problem of promoting heavily used folios from slower to faster
memory, while the other aims to improve the kernel's handling of nodes
containing special memory installed for a specific purpose.