[PATCH v5 19/28] mm: page_alloc: preserve PG_zeroed in page_del_and_expand

["Michael S. Tsirkin" <mst@redhat.com>]

Propagate PG_zeroed through buddy splits in page_del_and_expand()
and try_to_claim_block().  When a zeroed high-order page is split
to satisfy a smaller allocation, the sub-pages placed back on the
free lists keep PG_zeroed.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh
---
 mm/page_alloc.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 63b7f396ff30..5c6ab949855a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1743,7 +1743,8 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
  * -- nyc
  */
 static inline unsigned int expand(struct zone *zone, struct page *page, int low,
-				  int high, int migratetype, bool reported)
+				  int high, int migratetype, bool reported,
+				  bool zeroed)
 {
 	unsigned int size = 1 << high;
 	unsigned int nr_added = 0;
@@ -1774,6 +1775,8 @@ static inline unsigned int expand(struct zone *zone, struct page *page, int low,
 		 */
 		if (reported)
 			__SetPageReported(&page[size]);
+		if (zeroed)
+			__SetPageZeroed(&page[size]);
 	}
 
 	return nr_added;
@@ -1785,10 +1788,12 @@ static __always_inline void page_del_and_expand(struct zone *zone,
 {
 	int nr_pages = 1 << high;
 	bool was_reported = page_reported(page);
+	bool was_zeroed = PageZeroed(page);
 
 	__del_page_from_free_list(page, zone, high, migratetype);
 
-	nr_pages -= expand(zone, page, low, high, migratetype, was_reported);
+	nr_pages -= expand(zone, page, low, high, migratetype, was_reported,
+			   was_zeroed);
 	account_freepages(zone, -nr_pages, migratetype);
 }
 
@@ -2366,11 +2371,12 @@ try_to_claim_block(struct zone *zone, struct page *page,
 	if (current_order >= pageblock_order) {
 		unsigned int nr_added;
 		bool was_reported = page_reported(page);
+		bool was_zeroed = PageZeroed(page);
 
 		del_page_from_free_list(page, zone, current_order, block_type);
 		change_pageblock_range(page, current_order, start_type);
 		nr_added = expand(zone, page, order, current_order, start_type,
-				  was_reported);
+				  was_reported, was_zeroed);
 		account_freepages(zone, nr_added, start_type);
 		return page;
 	}
-- 
MST