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mm/khugepaged: fix PMD collapse swap PTE accounting

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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: 90ed32d000 ("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>
This commit is contained in:
Lance Yang 2026-06-09 20:04:43 +08:00 committed by Andrew Morton
parent cae43f22de
commit c85418be96
1 changed files with 8 additions and 0 deletions
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8
mm/khugepaged.c
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@ -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;