On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand <david@xxxxxxxxxx> 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 You can take my collapse_max_ptes_none() function in this series and rework it for the larger sysctl work you are doing. Cheers, -- Nico > > In between we will have more flexibility on how to set these values. > > Likely 9 will be around 50% to not even motivate the user to set > something that does not make sense (creep). > > Of course, the old parameter will have to stick around in compat mode. > > -- > Cheers > > David / dhildenb >
