[$] Ways to reclaim unused page-table pages

One of the memory-management subsystem's most important jobs is reclaimingunused (or little-used) memory so that it can be put to better use. When it comes toone 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 asession on the problems posed by the lack of page-table reclaim andexplored options for improving the situation.