Index: share/man/man9/vm_page_alloc.9
===================================================================
--- share/man/man9/vm_page_alloc.9
+++ share/man/man9/vm_page_alloc.9
@@ -26,7 +26,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 16, 2016
+.Dd February 19, 2021
 .Dt VM_PAGE_ALLOC 9
 .Os
 .Sh NAME
@@ -103,11 +103,7 @@
 .It Dv VM_ALLOC_WIRED
 The returned page will be wired.
 .It Dv VM_ALLOC_ZERO
-Indicate a preference for a pre-zeroed page.
-There is no guarantee that the returned page will be zeroed, but it
-will have the
-.Dv PG_ZERO
-flag set if it is zeroed.
+Zero-fill the contents of the page.
 .El
 .El
 .Sh RETURN VALUES
Index: sys/vm/vm_page.c
===================================================================
--- sys/vm/vm_page.c
+++ sys/vm/vm_page.c
@@ -1949,6 +1949,19 @@
 	return (0);
 }
 
+static void
+vm_page_alloc_zero(vm_page_t m, int req)
+{
+	if ((req & VM_ALLOC_ZERO) != 0) {
+		if ((m->flags & PG_ZERO) != 0) {
+			VM_CNT_INC(v_ozfod);
+		} else {
+			pmap_zero_page(m);
+			VM_CNT_INC(v_zfod);
+		}
+	}
+}
+
 /*
  *	vm_page_alloc:
  *
@@ -1969,9 +1982,10 @@
  *	VM_ALLOC_NODUMP		do not include the page in a kernel core dump
  *	VM_ALLOC_NOOBJ		page is not associated with an object and
  *				should not be exclusive busy
+ *
  *	VM_ALLOC_SBUSY		shared busy the allocated page
  *	VM_ALLOC_WIRED		wire the allocated page
- *	VM_ALLOC_ZERO		prefer a zeroed page
+ *	VM_ALLOC_ZERO		return a zeroed page
  */
 vm_page_t
 vm_page_alloc(vm_object_t object, vm_pindex_t pindex, int req)
@@ -2144,10 +2158,9 @@
 	vm_page_alloc_check(m);
 
 	/*
-	 * Initialize the page.  Only the PG_ZERO flag is inherited.
+	 * Initialize the page.  Zero its contents if so requested.
 	 */
-	if ((req & VM_ALLOC_ZERO) != 0)
-		flags |= (m->flags & PG_ZERO);
+	vm_page_alloc_zero(m, req);
 	if ((req & VM_ALLOC_NODUMP) != 0)
 		flags |= PG_NODUMP;
 	m->flags = flags;
@@ -2175,7 +2188,8 @@
 			KASSERT(m->object == NULL, ("page %p has object", m));
 			m->oflags = VPO_UNMANAGED;
 			m->busy_lock = VPB_UNBUSIED;
-			/* Don't change PG_ZERO. */
+			if ((req & VM_ALLOC_ZERO) != 0)
+				m->flags |= PG_ZERO;
 			vm_page_free_toq(m);
 			if (req & VM_ALLOC_WAITFAIL) {
 				VM_OBJECT_WUNLOCK(object);
@@ -2232,7 +2246,7 @@
  *				should not be exclusive busy
  *	VM_ALLOC_SBUSY		shared busy the allocated page
  *	VM_ALLOC_WIRED		wire the allocated page
- *	VM_ALLOC_ZERO		prefer a zeroed page
+ *	VM_ALLOC_ZERO		return a zeroed page
  */
 vm_page_t
 vm_page_alloc_contig(vm_object_t object, vm_pindex_t pindex, int req,
@@ -2334,11 +2348,9 @@
 	}
 
 	/*
-	 * Initialize the pages.  Only the PG_ZERO flag is inherited.
+	 * Initialize the pages.  Zero their contents if so requested.
 	 */
 	flags = 0;
-	if ((req & VM_ALLOC_ZERO) != 0)
-		flags = PG_ZERO;
 	if ((req & VM_ALLOC_NODUMP) != 0)
 		flags |= PG_NODUMP;
 	oflags = object == NULL || (object->flags & OBJ_UNMANAGED) != 0 ?
@@ -2357,8 +2369,9 @@
 			memattr = object->memattr;
 	}
 	for (m = m_ret; m < &m_ret[npages]; m++) {
+		vm_page_alloc_zero(m, req);
+		m->flags = flags;
 		m->a.flags = 0;
-		m->flags = (m->flags | PG_NODUMP) & flags;
 		m->busy_lock = busy_lock;
 		if ((req & VM_ALLOC_WIRED) != 0)
 			m->ref_count = 1;
@@ -2377,7 +2390,8 @@
 						m->ref_count = 0;
 					m->oflags = VPO_UNMANAGED;
 					m->busy_lock = VPB_UNBUSIED;
-					/* Don't change PG_ZERO. */
+					if ((req & VM_ALLOC_ZERO) != 0)
+						m->flags |= PG_ZERO;
 					vm_page_free_toq(m);
 				}
 				if (req & VM_ALLOC_WAITFAIL) {
@@ -2434,7 +2448,7 @@
  *	VM_ALLOC_COUNT(number)	the number of additional pages that the caller
  *				intends to allocate
  *	VM_ALLOC_WIRED		wire the allocated page
- *	VM_ALLOC_ZERO		prefer a zeroed page
+ *	VM_ALLOC_ZERO		return a zeroed page
  */
 vm_page_t
 vm_page_alloc_freelist(int freelist, int req)
@@ -2458,7 +2472,6 @@
 {
 	struct vm_domain *vmd;
 	vm_page_t m;
-	u_int flags;
 
 	m = NULL;
 	vmd = VM_DOMAIN(domain);
@@ -2480,13 +2493,11 @@
 	vm_page_alloc_check(m);
 
 	/*
-	 * Initialize the page.  Only the PG_ZERO flag is inherited.
+	 * Initialize the pages.  Zero their contents if so requested.
 	 */
+	vm_page_alloc_zero(m, req);
+	m->flags = 0;
 	m->a.flags = 0;
-	flags = 0;
-	if ((req & VM_ALLOC_ZERO) != 0)
-		flags = PG_ZERO;
-	m->flags &= flags;
 	if ((req & VM_ALLOC_WIRED) != 0) {
 		vm_wire_add(1);
 		m->ref_count = 1;