[Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order

[Wei Yang <richard.weiyang@gmail.com>]

We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
While if the order is less than start_pfn aligned order, we would get the
same pfn and do the same check again.

Iterate from start_pfn aligned order to reduce duplicated work.

Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>

---
v2: add comment on assignment of order
---
 mm/page_alloc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 07d79ae557f8..5d9ceca869e5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
 /* Look for a buddy that straddles start_pfn */
 static unsigned long find_large_buddy(unsigned long start_pfn)
 {
-	int order = 0;
+	/*
+	 * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
+	 * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
+	 * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
+	 * the starting order does not matter.
+	 */
+	int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
 	struct page *page;
 	unsigned long pfn = start_pfn;
 
-- 
2.34.1