From: Lance Yang <lance.yang@linux.dev>
Date: Tue, 9 Jun 2026 12:04:43 +0000 (+0800)
Subject: mm/khugepaged: fix PMD collapse swap PTE accounting
X-Git-Tag: v7.2-rc1~59^2~98
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=c85418be96666be9d6127448baad95ca404ee34e;p=thirdparty%2Fkernel%2Flinux.git

mm/khugepaged: fix PMD collapse swap PTE accounting

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.

Link: https://lore.kernel.org/20260609120443.71864-1-lance.yang@linux.dev
Fixes: 90ed32d00054 ("mm/khugepaged: introduce mTHP collapse support")
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Acked-by: Nico Pache <npache@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6de1a148222a0..33ab5d12e1bc7 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1502,6 +1502,14 @@ 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;