Re: [PATCH mm-unstable v18 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Lance Yang]

On Fri, May 22, 2026 at 09:00:07AM -0600, Nico Pache wrote:
>There are cases where, if an attempted collapse fails, all subsequent
>orders are guaranteed to also fail. Avoid these collapse attempts by
>bailing out early.
>
>Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
>Acked-by: Usama Arif <usama.arif@xxxxxxxxx>
>Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
>Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
>---
> mm/khugepaged.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
>diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>index d3d7db8be26c..15b7298bc225 100644
>--- a/mm/khugepaged.c
>+++ b/mm/khugepaged.c
>@@ -1535,9 +1535,31 @@ static int mthp_collapse(struct mm_struct *mm, struct vm_area_struct *vma,
> 			collapse_address = address + offset * PAGE_SIZE;
> 			ret = collapse_huge_page(mm, collapse_address, referenced,
> 						 unmapped, cc, order);
>-			if (ret == SCAN_SUCCEED) {
>+
>+			switch (ret) {
>+			/* Cases where we continue to next collapse candidate */
>+			case SCAN_SUCCEED:
> 				collapsed += nr_ptes;
>+				fallthrough;
>+			case SCAN_PTE_MAPPED_HUGEPAGE:
> 				continue;
>+			/* Cases where lower orders might still succeed */
>+			case SCAN_LACK_REFERENCED_PAGE:
>+			case SCAN_EXCEED_NONE_PTE:
>+			case SCAN_EXCEED_SWAP_PTE:
>+			case SCAN_EXCEED_SHARED_PTE:
>+			case SCAN_PAGE_LOCK:
>+			case SCAN_PAGE_COUNT:
>+			case SCAN_PAGE_NULL:
>+			case SCAN_DEL_PAGE_LRU:
>+			case SCAN_PTE_NON_PRESENT:
>+			case SCAN_PTE_UFFD_WP:
>+			case SCAN_ALLOC_HUGE_PAGE_FAIL:

Nit: shouldn't SCAN_CGROUP_CHARGE_FAIL go with SCAN_ALLOC_HUGE_PAGE_FAIL
here?

If charging the current order fails, a smaller order might still fit :)

Cheers, Lance

>+			case SCAN_PAGE_LAZYFREE:
>+				goto next_order;
>+			/* Cases where no further collapse is possible */
>+			default:
>+				return collapsed;
> 			}
> 		}
> 
>-- 
>2.54.0
>
>