[PATCH v11 13/15] 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.

Signed-off-by: Nico Pache <npache@redhat.com>
---
 mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 8455a02dc3d6..ead07ccac351 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1435,10 +1435,39 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address,
 			ret = collapse_huge_page(mm, address, referenced,
 						 unmapped, cc, mmap_locked,
 						 order, offset);
-			if (ret == SCAN_SUCCEED) {
+
+			/*
+			 * Analyze failure reason to determine next action:
+			 * - goto next_order: try smaller orders in same region
+			 * - continue: try other regions at same order
+			 * - break: stop all attempts (system-wide failure)
+			 */
+			switch (ret) {
+			/* Cases were we should continue to the next region */
+			case SCAN_SUCCEED:
 				collapsed += 1UL << order;
+				fallthrough;
+			case SCAN_PTE_MAPPED_HUGEPAGE:
 				continue;
+			/* Cases were 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_LRU:
+			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:
+				goto next_order;
+			/* All other cases should stop collapse attempts */
+			default:
+				break;
 			}
+			break;
 		}
 
 next_order:
-- 
2.51.0