[$] Proactive reclaim for tiered memory and more

Memory reclaim in Linux is largely a reactive practice; the kernel tries to
find memory it can repurpose in response to the amount of free memory
falling too low. Developers have often wondered if a proactive reclaim
mechanism might lead to better performance, for some workloads at least,
and optimal use of tiered-memory systems will likely require more active
reclamation of memory as well. At the 2022 Linux Storage,
Filesystem, Memory-management and BPF Summit (LSFMM), Davidlohr Bueso
led a brief session on the topic.