We do not necessarily need to map/unmap pages to zero parts of them.

On systems where physical memory is also direct mapped (alpha, sparc,
ia64 etc) this is slightly harmful.

sys/vm/vm_page.c

@@ -456,6 +456,18 @@ vm_page_zero_fill(vm_page_t m)
 	return (TRUE);
 }
 
+/*
+ *	vm_page_zero_fill_area:
+ *
+ *	Like vm_page_zero_fill but only fill the specified area.
+ */
+boolean_t
+vm_page_zero_fill_area(vm_page_t m, int off, int size)
+{
+	pmap_zero_page_area(m, off, size);
+	return (TRUE);
+}
+
 /*
  *	vm_page_copy:
  *

sys/vm/vm_page.h

@@ -327,6 +327,7 @@ void vm_page_hold(vm_page_t mem);
 void vm_page_unhold(vm_page_t mem);
 void vm_page_protect(vm_page_t mem, int prot);
 boolean_t vm_page_zero_fill(vm_page_t m);
+boolean_t vm_page_zero_fill_area(vm_page_t m, int off, int len);
 void vm_page_copy(vm_page_t src_m, vm_page_t dest_m);
 void vm_page_free(vm_page_t m);
 void vm_page_free_zero(vm_page_t m);

sys/vm/vnode_pager.c

@@ -305,7 +305,6 @@ vnode_pager_setsize(vp, nsize)
 		 * it can screw up NFS reads, so we don't allow the case.
 		 */
 		if (nsize & PAGE_MASK) {
-			vm_offset_t kva;
 			vm_page_t m;
 
 			m = vm_page_lookup(object, OFF_TO_IDX(nsize));
@@ -317,9 +316,7 @@ vnode_pager_setsize(vp, nsize)
 				 * Clear out partial-page garbage in case
 				 * the page has been mapped.
 				 */
-				kva = vm_pager_map_page(m);
-				bzero((caddr_t)kva + base, size);
-				vm_pager_unmap_page(kva);
+				vm_page_zero_fill_area(m, base, size);
 
 				/*
 				 * XXX work around SMP data integrity race