On Fri, Sep 12, 2025 at 05:31:51PM -0600, Nico Pache wrote: > On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand <[email protected]> wrote: > > > > On 12.09.25 14:19, Kiryl Shutsemau wrote: > > > 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. > > > > > > > I am all for not adding any more ugliness on top of all the ugliness we > > added in the past. > > > > I will soon propose deprecating that parameter in favor of something > > that makes a bit more sense. > > > > In essence, we'll likely have an "eagerness" parameter that ranges from > > 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if > > not all is populated". > Hi David, > > Do you have any reason for 0-10, I'm guessing these will map to > different max_ptes_none values. > I suggest 0-5, mapping to 0,32,64,128,255,511
That's too x86-64 specific. And the whole idea is not to map to directly, but give kernel wiggle room to play. -- Kiryl Shutsemau / Kirill A. Shutemov
