[$] Facing down mapcount madness

The pagestructure is a complicated beast, but some parts of it are moreintimidating than others. The mapcount field is one of thescarier parts. It allegedly records the number of references to the pagein page tables, but, as David Hildenbrand described during thememory-management track at the 2024 Linux Storage,Filesystem, Memory Management, and BPF Summit, things are morecomplicated than that. Few people truly understand the semantics of thisfield, but the situation will hopefully get better over time.