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

[Wei Yang <richard.weiyang@gmail.com>]

On Tue, Jun 09, 2026 at 08:04:43PM +0800, Lance Yang wrote:
>From: Lance Yang <lance.yang@linux.dev>
>
>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@linux.dev>
>---
>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.
>
>[1] https://lore.kernel.org/linux-mm/CAA1CXcD7WAiA1b9GTLAuNZ+kHaFx0SzZwpBkqAZ=s+RHsTUaow@mail.gmail.com/
>[2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@redhat.com/
>
> 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;
>+

Nice catch, I didn't spot it.

Reviewed-by: Wei Yang <richard.weiyang@gmail.com>

> 		if (nr_occupied_ptes >= nr_ptes - max_ptes_none) {
> 			enum scan_result ret;
> 
>-- 
>2.49.0
>

-- 
Wei Yang
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