[PATCH mm-unstable v17 12/14] mm/khugepaged: avoid unnecessary mTHP collapse attempts

[Nico Pache <npache@redhat.com>]

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 <npache@redhat.com>
---
 mm/khugepaged.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 39bf7ea8a6e8..f0ae02936638 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1531,9 +1531,31 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address,
 			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:
+			case SCAN_PAGE_LAZYFREE:
+				goto next_order;
+			/* Cases where no further collapse is possible */
+			default:
+				return collapsed;
 			}
 		}
 
-- 
2.54.0