Index: head/sys/vm/vm_page.c
===================================================================
--- head/sys/vm/vm_page.c
+++ head/sys/vm/vm_page.c
@@ -1324,22 +1324,17 @@
 vm_page_t
 vm_page_replace(vm_page_t mnew, vm_object_t object, vm_pindex_t pindex)
 {
-	vm_page_t mold, mpred;
+	vm_page_t mold;
 
 	VM_OBJECT_ASSERT_WLOCKED(object);
+	KASSERT(mnew->object == NULL,
+	    ("vm_page_replace: page already in object"));
 
 	/*
 	 * This function mostly follows vm_page_insert() and
 	 * vm_page_remove() without the radix, object count and vnode
 	 * dance.  Double check such functions for more comments.
 	 */
-	mpred = vm_radix_lookup(&object->rtree, pindex);
-	KASSERT(mpred != NULL,
-	    ("vm_page_replace: replacing page not present with pindex"));
-	mpred = TAILQ_PREV(mpred, respgs, listq);
-	if (mpred != NULL)
-		KASSERT(mpred->pindex < pindex,
-		    ("vm_page_insert_after: mpred doesn't precede pindex"));
 
 	mnew->object = object;
 	mnew->pindex = pindex;
@@ -1347,17 +1342,17 @@
 	KASSERT(mold->queue == PQ_NONE,
 	    ("vm_page_replace: mold is on a paging queue"));
 
-	/* Detach the old page from the resident tailq. */
+	/* Keep the resident page list in sorted order. */
+	TAILQ_INSERT_AFTER(&object->memq, mold, mnew, listq);
 	TAILQ_REMOVE(&object->memq, mold, listq);
 
 	mold->object = NULL;
 	vm_page_xunbusy(mold);
 
-	/* Insert the new page in the resident tailq. */
-	if (mpred != NULL)
-		TAILQ_INSERT_AFTER(&object->memq, mpred, mnew, listq);
-	else
-		TAILQ_INSERT_HEAD(&object->memq, mnew, listq);
+	/*
+	 * The object's resident_page_count does not change because we have
+	 * swapped one page for another, but OBJ_MIGHTBEDIRTY.
+	 */
 	if (pmap_page_is_write_mapped(mnew))
 		vm_object_set_writeable_dirty(object);
 	return (mold);