[$] Ways to reclaim unused page-table pages

One of the memory-management subsystem's most important jobs is reclaiming
unused (or
little-used) memory so that it can be put to better use. When it comes to
one of the core memory-management data structures — page tables — though,
this subsystem often falls down on the job. At the 2022 Linux Storage,
Filesystem, Memory-management and BPF Summit (LSFMM), David Hildenbrand led a
session on the problems posed by the lack of page-table reclaim and
explored options for improving the situation.