[PATCH v9 36/37] mm: balloon: use put_page_zeroed for zeroed balloon pages

[Michael S. Tsirkin]

When a balloon page marked PageZeroed is freed during migration,
use put_page_zeroed() to propagate the zeroed hint to the buddy
allocator. Previously the hint was silently lost via plain put_page().

No page has PageZeroed set yet; the next patch
(VIRTIO_BALLOON_F_DEVICE_INIT_ON_INFLATE) will set it on
pages the host has zeroed during inflate.
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 mm/balloon.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/balloon.c b/mm/balloon.c
index 96a8f1e20bc6..6c9dd8ab0c5d 100644
--- a/mm/balloon.c
+++ b/mm/balloon.c
@@ -324,7 +324,15 @@ static int balloon_page_migrate(struct page *newpage, struct page *page,
 	balloon_page_finalize(page);
 	spin_unlock_irqrestore(&balloon_pages_lock, flags);
 
-	put_page(page);
+	if (PageZeroed(page)) {
+		/* Atomic to serialize with memory_failure's
+		 * TestSetPageHWPoison; not under zone->lock here.
+		 */
+		ClearPageZeroed(page);
+		put_page_zeroed(page);
+	} else {
+		put_page(page);
+	}
 
 	return 0;
 }
-- 
MST