[PATCH v11 13/15] khugepaged: avoid unnecessary mTHP collapse attempts

[Nico Pache]

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 <npa...@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