[PATCH v8 33/37] mm: add free_frozen_pages_zeroed

[Michael S. Tsirkin]

Add free_frozen_pages_zeroed(page, order) to free a frozen page
while marking it as zeroed, so the next allocation can skip
redundant zeroing.

An FPI_ZEROED internal flag carries the hint through the free path.
PageZeroed is set after __free_pages_prepare() clears all flags,
so the hint survives on the free list.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 include/linux/gfp.h |  1 +
 mm/internal.h       |  1 -
 mm/page_alloc.c     | 29 ++++++++++++++++++++++++++++-
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 73109d4e31a4..d24b61e45861 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -384,6 +384,7 @@ __meminit void *alloc_pages_exact_nid_noprof(int nid, size_t size, gfp_t gfp_mas
 extern void __free_pages(struct page *page, unsigned int order);
 extern void free_pages_nolock(struct page *page, unsigned int order);
 extern void free_pages(unsigned long addr, unsigned int order);
+void free_frozen_pages_zeroed(struct page *page, unsigned int order);
 
 #define __free_page(page) __free_pages((page), 0)
 #define free_page(addr) free_pages((addr), 0)
diff --git a/mm/internal.h b/mm/internal.h
index fd910743ddc3..4af5e72742ba 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -938,7 +938,6 @@ struct page *__alloc_frozen_pages_noprof(gfp_t, unsigned int order, int nid,
 #define __alloc_frozen_pages(...) \
 	alloc_hooks(__alloc_frozen_pages_noprof(__VA_ARGS__))
 void free_frozen_pages(struct page *page, unsigned int order);
-void free_frozen_pages_zeroed(struct page *page, unsigned int order);
 void free_unref_folios(struct folio_batch *fbatch);
 
 #ifdef CONFIG_NUMA
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 702fa2ced1e1..399e2038dcd9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -90,6 +90,13 @@ typedef int __bitwise fpi_t;
 /* Free the page without taking locks. Rely on trylock only. */
 #define FPI_TRYLOCK		((__force fpi_t)BIT(2))
 
+/*
+ * The page contents are known to be zero (e.g., the host zeroed them
+ * during balloon deflate).  Set PageZeroed after free so the next
+ * allocation can skip redundant zeroing.
+ */
+#define FPI_ZEROED		((__force fpi_t)BIT(3))
+
 /* prevent >1 _updater_ of zone percpu pageset ->high and ->batch fields */
 static DEFINE_MUTEX(pcp_batch_high_lock);
 #define MIN_PERCPU_PAGELIST_HIGH_FRACTION (8)
@@ -1599,8 +1606,19 @@ static void __free_pages_ok(struct page *page, unsigned int order,
 	unsigned long pfn = page_to_pfn(page);
 	struct zone *zone = page_zone(page);
 
-	if (__free_pages_prepare(page, order, fpi_flags))
+	if (__free_pages_prepare(page, order, fpi_flags)) {
+		/*
+		 * Only mark as zeroed if the page is actually still zero.
+		 * kernel_poison_pages() overwrites with PAGE_POISON (0xAA)
+		 * when CONFIG_PAGE_POISONING is enabled.  With init_on_free,
+		 * kernel_init_pages() re-zeroes after poisoning, so the
+		 * page is still zero.
+		 */
+		if ((fpi_flags & FPI_ZEROED) &&
+		    (!page_poisoning_enabled_static() || want_init_on_free()))
+			__SetPageZeroed(page);
 		free_one_page(zone, page, pfn, order, fpi_flags);
+	}
 }
 
 void __meminit __free_pages_core(struct page *page, unsigned int order,
@@ -3021,6 +3039,9 @@ static void __free_frozen_pages(struct page *page, unsigned int order,
 	if (!__free_pages_prepare(page, order, fpi_flags))
 		return;
 
+	if (fpi_flags & FPI_ZEROED)
+		__SetPageZeroed(page);
+
 	/*
 	 * We only track unmovable, reclaimable and movable on pcp lists.
 	 * Place ISOLATE pages on the isolated list because they are being
@@ -3059,6 +3080,12 @@ void free_frozen_pages(struct page *page, unsigned int order)
 	__free_frozen_pages(page, order, FPI_NONE);
 }
 
+void free_frozen_pages_zeroed(struct page *page, unsigned int order)
+{
+	__free_frozen_pages(page, order, FPI_ZEROED);
+}
+EXPORT_SYMBOL(free_frozen_pages_zeroed);
+
 void free_frozen_pages_nolock(struct page *page, unsigned int order)
 {
 	__free_frozen_pages(page, order, FPI_TRYLOCK);
-- 
MST