[RFC PATCH 06/57] mm/collapse: move the smallest collapse order to collapse.h

[Kiryl Shutsemau]

From: "Kiryl Shutsemau (Meta)" <kas@xxxxxxxxxx>

The floor on the order a collapse will build is a property of collapse,
not of khugepaged.  MADV_COLLAPSE reaches the same code and is held to
the same floor.

Move KHUGEPAGED_MIN_MTHP_ORDER to the shared header as
COLLAPSE_MIN_MTHP_ORDER.  Preparation for the collapse engine, which
sizes its per-window arrays from the same floor.

No functional change intended.

Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
---
 mm/collapse.h   | 3 +++
 mm/khugepaged.c | 6 ++----
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/collapse.h b/mm/collapse.h
index 44f52ea5bbb8..1e969292edcb 100644
--- a/mm/collapse.h
+++ b/mm/collapse.h
@@ -7,6 +7,9 @@
 #include <linux/pgtable.h>
 #include <linux/types.h>
 
+/* The smallest order a collapse will build, and so the finest window it cuts */
+#define COLLAPSE_MIN_MTHP_ORDER		2
+
 enum scan_result {
 	SCAN_FAIL,
 	SCAN_SUCCEED,
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index eebc044a930e..f31689bf75a6 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -67,8 +67,6 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
 
 static struct kmem_cache *mm_slot_cache __ro_after_init;
 
-#define KHUGEPAGED_MIN_MTHP_ORDER	2
-
 /**
  * struct khugepaged_scan - cursor for scanning
  * @mm_head: the head of the mm list to scan
@@ -1540,8 +1538,8 @@ static enum scan_result mthp_collapse(struct mm_struct *mm,
 		 * any smaller order enabled. When at the smallest order
 		 * we must always move to the next offset.
 		 */
-		if (order > KHUGEPAGED_MIN_MTHP_ORDER &&
-			(enabled_orders & GENMASK(order - 1, 0))) {
+		if (order > COLLAPSE_MIN_MTHP_ORDER &&
+		    (enabled_orders & GENMASK(order - 1, 0))) {
 			order--;
 			continue;
 		}
-- 
2.54.0