[$] Page aging with hardware counters

The memory-management subsystem has the unenviable task of trying topredict which pages of memory will be needed in the near future. Sincepredictions tend to be difficult, the code relies heavily on the heuristicthat memory used in the recent past is likely to be used again in the nearfuture. However, even knowing which memory has been recently used can be achallenge. At the 2023 Linux Storage,Filesystem, Memory-Management and BPF Summit, Aneesh Kumar and Wei Xu,both presenting remotely,discussed some ways to use the increasingly capable hardware counters thatare provided by current and upcoming CPUs.