linux-kernel - [PATCH v10 08/13] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <20250819134205.622806-9-npache@redhat.com>
Date: Tue, 19 Aug 2025 07:42:00 -0600
From: Nico Pache <npache@...hat.com>
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Subject: [PATCH v10 08/13] khugepaged: avoid unnecessary mTHP collapse attempts

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@...hat.com>
---
 mm/khugepaged.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 6a4cf7e4a7cc..7d9b5100bea1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1389,10 +1389,39 @@ 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 * KHUGEPAGED_MIN_MTHP_NR);
-			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 += (1 << order);
+			case SCAN_PAGE_RO:
+			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:
-- 
2.50.1