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

Nico Pache
2025-08-19T13:45:37Z

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 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