[PATCH v7 25/31] mm: page_alloc: propagate PG_zeroed in split_large_buddy

[Michael S. Tsirkin]

When splitting a large buddy page, propagate the PG_zeroed flag
to each sub-page before freeing it.  __free_pages_prepare clears
all flags (including PG_zeroed), so the flag must be re-set on
each fragment after the split.  This ensures that the buddy merge
logic can see PG_zeroed on pages that were part of a larger
zeroed block.

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

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 468e8bde7d34..ce43f5a3dbaa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1523,6 +1523,7 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 			      unsigned long pfn, int order, fpi_t fpi)
 {
 	unsigned long end = pfn + (1 << order);
+	bool zeroed = PageZeroed(page);
 
 	VM_WARN_ON_ONCE(!IS_ALIGNED(pfn, 1 << order));
 	/* Caller removed page from freelist, buddy info cleared! */
@@ -1534,6 +1535,8 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 	do {
 		int mt = get_pfnblock_migratetype(page, pfn);
 
+		if (zeroed)
+			__SetPageZeroed(page);
 		__free_one_page(page, pfn, zone, order, mt, fpi);
 		pfn += 1 << order;
 		if (pfn == end)
-- 
MST