Making DAMON monitor only writes

DAMON is a Linux kernel subsystem that provides a framework for data access monitoring and the monitoring results based system operations. Nowaday DAMON counts as access reads and writes to a memory page.

I proposed the patch [1] to add an option to monitor only write accesses. This patch also adds the actions mergeable and unmergeable to damos schemes, that will call madvise with flags MADV_MERGEABLE and MADV_UNMERGEABLE, repectively.

The idea is to use the mergeable action in cold areas, and the unmergeable in hot areas, to avoid KSM overhead.

[1] https://marc.info/?l=linux-kernel&m=164389387832384