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

On 2026/6/1 04:02, David Hildenbrand (Arm) wrote:

On 5/31/26 09:31, Lance Yang wrote:

quoted

On Fri, May 22, 2026 at 09:00:07AM -0600, Nico Pache wrote:

quoted

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@kernel.org>
Acked-by: Usama Arif <usama.arif@linux.dev>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nico Pache <redacted>
---
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 :)

I think the reasoning was here, that if we are already that close to our mem
limit, we should just give up instead of trying to squeeze it in .. :)

Fair point. Just a nit, nevermind :)