Re: [PATCH] mm/thp: support mTHP in thp_underused()

[Nico Pache]

On Wed, Jul 15, 2026 at 12:28 AM Ye Liu <ye.liu@xxxxxxxxx> wrote:
>
> From: Ye Liu <liuye@xxxxxxxxxx>
>
> When khugepaged_max_ptes_none is configured to a non-default value,
> scale it for non-PMD-mappable folios to match the logic in
> collapse_max_ptes_none(), so mTHP can be classified as underused
> instead of being silently skipped due to unscaled PMD-sized thresholds.

IIUC we don't actually add non-PMD folios to the deferred split list
and only operate on fully mapped folios, so consequently, we never try
to shrink them with the underused shrinker.

Let me know if im missing something!

Cheers,
-- Nico

>
> Signed-off-by: Ye Liu <liuye@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 25612af72dca..c642bec967fa 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4489,24 +4489,29 @@ static unsigned long deferred_split_count(struct shrinker *shrink,
>  static bool thp_underused(struct folio *folio)
>  {
>         int num_zero_pages = 0, num_filled_pages = 0;
> +       unsigned int max_ptes_none = khugepaged_max_ptes_none;
> +       unsigned int folio_nr = folio_nr_pages(folio);
>         int i;
>
> -       if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> +       if (!folio_test_pmd_mappable(folio)) {
> +               if (max_ptes_none == HPAGE_PMD_NR - 1)
> +                       max_ptes_none = folio_nr - 1;
> +               else if (max_ptes_none)
> +                       return false;
> +       }
> +
> +       if (max_ptes_none == folio_nr - 1)
>                 return false;
>
>         if (folio_contain_hwpoisoned_page(folio))
>                 return false;
>
> -       for (i = 0; i < folio_nr_pages(folio); i++) {
> +       for (i = 0; i < folio_nr; i++) {
>                 if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) {
> -                       if (++num_zero_pages > khugepaged_max_ptes_none)
> +                       if (++num_zero_pages > max_ptes_none)
>                                 return true;
>                 } else {
> -                       /*
> -                        * Another path for early exit once the number
> -                        * of non-zero filled pages exceeds threshold.
> -                        */
> -                       if (++num_filled_pages >= HPAGE_PMD_NR - khugepaged_max_ptes_none)
> +                       if (++num_filled_pages >= folio_nr - max_ptes_none)
>                                 return false;
>                 }
>         }
> --
> 2.43.0
>