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

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