Re: [PATCH mm-unstable v19 07/14] mm/khugepaged: skip collapsing mTHP to smaller orders

[Lance Yang]

On Fri, Jun 05, 2026 at 10:14:14AM -0600, Nico Pache wrote:
>khugepaged may try to collapse a mTHP to a folio of equal or smaller size,
>possibly resulting in a partially mapped source folio, which is undesired.
>Skip these cases until we have a way to check if its ok to collapse to a
>smaller mTHP size (like in the case of a partially mapped folio). This
>check is not done during the scan phase as the current collapse order is
>unknown at that time.
>
>This patch is inspired by Dev Jain's work on khugepaged mTHP support [1].
>
>[1] https://lore.kernel.org/lkml/20241216165105.56185-11-dev.jain@xxxxxxx/
>
>Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
>Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>Acked-by: David Hildenbrand (arm) <david@xxxxxxxxxx>
>Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
>Co-developed-by: Dev Jain <dev.jain@xxxxxxx>
>Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
>Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
>---
> mm/khugepaged.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index c2769d82a719..191e529c185c 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -697,6 +697,14 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
> 				goto out;
> 			}
> 		}
>+		/*
>+		 * TODO: In some cases of partially-mapped folios, we'd actually
>+		 * want to collapse.
>+		 */

Partially mapped folios can be handled later :)

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>

>+		if (!is_pmd_order(order) && folio_order(folio) >= order) {
>+			result = SCAN_PTE_MAPPED_HUGEPAGE;
>+			goto out;
>+		}
> 
> 		if (folio_test_large(folio)) {
> 			struct folio *f;
>-- 
>2.54.0
>
>