[PATCH RFC v4 21/22] mm: add put_page_zeroed and folio_put_zeroed

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

Add put_page_zeroed() / folio_put_zeroed() for callers that hold
a reference to a page known to be zeroed.

If this drops the last reference, the page goes through
__folio_put_zeroed() which calls free_frozen_pages_zeroed() so
the zeroed hint is preserved.  If someone else still holds a
reference, the hint is simply lost -- this is best-effort.

This is useful for balloon drivers during deflation: the host
has already zeroed the pages, and the balloon is typically the
sole owner.  But if the page happens to be shared, silently
dropping the hint is safe and avoids the need for callers to
check the refcount.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
---
 include/linux/mm.h | 12 ++++++++++++
 mm/swap.c          | 18 ++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 541d36e5e420..1c6bf82a967a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1640,6 +1640,7 @@ static inline struct folio *virt_to_folio(const void *x)
 }
 
 void __folio_put(struct folio *folio);
+void __folio_put_zeroed(struct folio *folio);
 
 void split_page(struct page *page, unsigned int order);
 void folio_copy(struct folio *dst, struct folio *src);
@@ -1817,6 +1818,17 @@ static inline void folio_put(struct folio *folio)
 		__folio_put(folio);
 }
 
+static inline void folio_put_zeroed(struct folio *folio)
+{
+	if (folio_put_testzero(folio))
+		__folio_put_zeroed(folio);
+}
+
+static inline void put_page_zeroed(struct page *page)
+{
+	folio_put_zeroed(page_folio(page));
+}
+
 /**
  * folio_put_refs - Reduce the reference count on a folio.
  * @folio: The folio.
diff --git a/mm/swap.c b/mm/swap.c
index bb19ccbece46..5d05a463b46a 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -94,7 +94,7 @@ static void page_cache_release(struct folio *folio)
 		unlock_page_lruvec_irqrestore(lruvec, flags);
 }
 
-void __folio_put(struct folio *folio)
+static void ___folio_put(struct folio *folio, bool zeroed)
 {
 	if (unlikely(folio_is_zone_device(folio))) {
 		free_zone_device_folio(folio);
@@ -109,10 +109,24 @@ void __folio_put(struct folio *folio)
 	page_cache_release(folio);
 	folio_unqueue_deferred_split(folio);
 	mem_cgroup_uncharge(folio);
-	free_frozen_pages(&folio->page, folio_order(folio));
+	if (zeroed)
+		free_frozen_pages_zeroed(&folio->page, folio_order(folio));
+	else
+		free_frozen_pages(&folio->page, folio_order(folio));
+}
+
+void __folio_put(struct folio *folio)
+{
+	___folio_put(folio, false);
 }
 EXPORT_SYMBOL(__folio_put);
 
+void __folio_put_zeroed(struct folio *folio)
+{
+	___folio_put(folio, true);
+}
+EXPORT_SYMBOL(__folio_put_zeroed);
+
 typedef void (*move_fn_t)(struct lruvec *lruvec, struct folio *folio);
 
 static void lru_add(struct lruvec *lruvec, struct folio *folio)
-- 
MST