Re: [PATCH v4 1/4] mm/huge_memory: scale thp_underused() to the folio size

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Nov 03, 2026 at 01:20:01PM -0800, Joanne Koong wrote:
> 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 folios, so it assumes that the
> folio has HPAGE_PMD_NR pages and compares the zero-filled page count
> directly against khugepaged_max_ptes_none. A later patch will queue
> anonymous mTHP folios onto the deferred split list as well, at which
> point that assumption will no longer hold.
> 
> Make thp_underused() folio-size-aware, and scale
> khugepaged_max_ptes_none proportionally to the number of pages in the
> folio (eg every folio tolerates the same fraction of zero-filled pages
> the user configured for a PMD-sized folio).
> 
> khugepaged deliberately doesn't scale intermediate values when
> collapsing to mTHP orders (to avoid the memory footprint creeping up
> through repeated collapses), but if/when collapse later gains support
> for intermediate values, a folio that khugepaged collapses won't then be
> split right back by the shrinker (and vice versa) since both collapse
> and split would use the same threshold for a given folio size.
> 
> There is no functional change for PMD-sized folios, as for those,
> nr_pages is HPAGE_PMD_NR.
> 
> Suggested-by: Usama Arif <usama.arif@linux.dev>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov