[PATCH RFC v2 03/18] mm: add PG_zeroed page flag for known-zero pages

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

Add PG_zeroed (aliased to PG_private) to track pages whose contents
are known to be zero.  Exclude __PG_ZEROED from PAGE_FLAGS_CHECK_AT_PREP
so the allocator does not BUG when encountering zeroed pages on the
free list.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Assisted-by: cursor-agent:GPT-5.4-xhigh
---
 include/linux/page-flags.h | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f7a0e4af0c73..f87ecb740e7f 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -157,6 +157,9 @@ enum pageflags {
 	 */
 	PG_fscache = PG_private_2,	/* page backed by cache */
 
+	/* Page contents are known to be zero (host-zeroed or balloon) */
+	PG_zeroed = PG_private,
+
 	/* XEN */
 	/* Pinned in Xen as a read-only pagetable page. */
 	PG_pinned = PG_owner_priv_1,
@@ -687,6 +690,14 @@ FOLIO_FLAG_FALSE(idle)
  */
 __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
 
+/*
+ * PageZeroed() tracks pages whose contents are known to be zero.
+ * Set on free-list pages by the balloon driver or page reporting.
+ * The allocator uses this to skip redundant zeroing.
+ */
+__PAGEFLAG(Zeroed, zeroed, PF_NO_COMPOUND)
+#define __PG_ZEROED (1UL << PG_zeroed)
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 PAGEFLAG(VmemmapSelfHosted, vmemmap_self_hosted, PF_ANY)
 #else
@@ -1209,7 +1220,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  * alloc-free cycle to prevent from reusing the page.
  */
 #define PAGE_FLAGS_CHECK_AT_PREP	\
-	((PAGEFLAGS_MASK & ~__PG_HWPOISON) | LRU_GEN_MASK | LRU_REFS_MASK)
+	((PAGEFLAGS_MASK & ~(__PG_HWPOISON | __PG_ZEROED)) | LRU_GEN_MASK | LRU_REFS_MASK)
 
 /*
  * Flags stored in the second page of a compound page.  They may overlap
-- 
MST