Re: [PATCH mm-unstable 1/1] mm/khugepaged: fix PMD collapse swap PTE accounting

[Nico Pache]

On Tue, Jun 9, 2026 at 6:05 AM Lance Yang <lance.yang@xxxxxxxxx> wrote:
>
> From: Lance Yang <lance.yang@xxxxxxxxx>
>
> mthp_collapse() uses mthp_present_ptes to decide whether a range has
> enough occupied PTEs to try collapse. Swap PTEs accepted by
> collapse_scan_pmd() are counted in unmapped, but are not represented in
> mthp_present_ptes.
>
> When lower orders are enabled, collapse_scan_pmd() relaxes max_ptes_none
> so the scan can cover the whole PMD and build the bitmap. mthp_collapse()
> then checks the PMD-order candidate using the bitmap.
>
> With max_ptes_none set to 0, a range with 511 present PTEs and one swap
> PTE no longer reaches collapse_huge_page(), even though PMD collapse can
> handle swap PTEs up to max_ptes_swap.
>
> Account unmapped PTEs only for PMD order. PMD collapse supports swap PTEs
> through max_ptes_swap, while lower-order mTHP collapse does not currently
> support non-present PTEs. Keep non-present PTEs out of the lower-order
> eligibility check.
>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---
> Sent separately, as discussed in [1], to spell out the PMD-order swap PTE
> case. Patch [2] is still only in mm-unstable, so no Fixes: tag.

Thanks for sending the fixup :)

Acked-by: Nico Pache <npache@xxxxxxxxxx>

>
> [1] https://lore.kernel.org/linux-mm/CAA1CXcD7WAiA1b9GTLAuNZ+kHaFx0SzZwpBkqAZ=s+RHsTUaow@xxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@xxxxxxxxxx/
>
>  mm/khugepaged.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b12187709f6d..617bca76db49 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1508,6 +1508,14 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
>                 nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset,
>                                                       offset + nr_ptes);
>
> +               /*
> +                * Swap PTEs accepted during the scan are counted in @unmapped,
> +                * not in the present-PTE bitmap. Account them for the PMD-order
> +                * candidate.
> +                */
> +               if (is_pmd_order(order))
> +                       nr_occupied_ptes += unmapped;
> +
>                 if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
>                         enum scan_result ret;
>
> --
> 2.49.0
>