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

[Lance Yang <lance.yang@linux.dev>]

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