拆分未充分使用的匿名mTHP · v4

thp_underused() decides whether a large folio on the deferred split list is underused and should be split so that its zero-filled subpages can be reclaimed. Today it only ever runs on PMD-sized folio

[mm/huge_memory: don’t queue folios when underused splitting is inactive] Every anonymous PMD-sized folio is put on the deferred split queue when it is first mapped, so that the shrinker can find it u

[mm/huge_memory: add a per-order counter for underused splits] thp_underused_split_page counts every folio that the deferred split shrinker splits for being underused. Today that is only ever a PMD-si

[mm: add anonymous mTHP folios to the deferred split list] Unlike for PMD-sized folios, an anonymous mTHP folio doesn’t get added to the deferred split list at fault or collapse time. As a result, a f