On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: > The following series provides khugepaged with the capability to collapse > anonymous memory regions to mTHPs. > > To achieve this we generalize the khugepaged functions to no longer depend > on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual > pages that are occupied (!none/zero). After the PMD scan is done, we do > binary recursion on the bitmap to find the optimal mTHP sizes for the PMD > range. The restriction on max_ptes_none is removed during the scan, to make > sure we account for the whole PMD range. When no mTHP size is enabled, the > legacy behavior of khugepaged is maintained. max_ptes_none will be scaled > by the attempted collapse order to determine how full a mTHP must be to be > eligible for the collapse to occur. If a mTHP collapse is attempted, but > contains swapped out, or shared pages, we don't perform the collapse. It is > now also possible to collapse to mTHPs without requiring the PMD THP size > to be enabled. > > When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on > 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for > mTHP collapses to prevent collapse "creep" behavior. This prevents > constantly promoting mTHPs to the next available size, which would occur > because a collapse introduces more non-zero pages that would satisfy the > promotion condition on subsequent scans.
Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. -- Kiryl Shutsemau / Kirill A. Shutemov
