[$] Managing multiple sources of page-hotness data

Knowing how frequently accessed a page of memory is (its 'hotness') is a
key input to many memory-management heuristics. Jonathan Cameron, in a
memory-management track at the 2025 Linux Storage, Filesystem,
Memory-Management, and BPF Summit, pointed out that the number of sources
of that kind of data is growing over time. He wanted to explore the
questions of what commonality exists between data from those sources, and
whether it makes sense to aggregate them all somehow.