diff --git a/sys/kern/uipc_shm.c b/sys/kern/uipc_shm.c
index cd1bf0b7142c..55362c5ad704 100644
--- a/sys/kern/uipc_shm.c
+++ b/sys/kern/uipc_shm.c
@@ -418,6 +418,7 @@ shm_dotruncate(struct shmfd *shmfd, off_t length)
 	vm_ooffset_t delta;
 	int base, rv;
 
+	KASSERT(length >= 0, ("shm_dotruncate: length < 0"));
 	object = shmfd->shm_object;
 	VM_OBJECT_WLOCK(object);
 	if (length == shmfd->shm_size) {
@@ -486,7 +487,7 @@ retry:
 				vm_pager_page_unswapped(m);
 			}
 		}
-		delta = ptoa(object->size - nobjsize);
+		delta = IDX_TO_OFF(object->size - nobjsize);
 
 		/* Toss in memory pages. */
 		if (nobjsize < object->size)
@@ -501,8 +502,8 @@ retry:
 		swap_release_by_cred(delta, object->cred);
 		object->charge -= delta;
 	} else {
-		/* Attempt to reserve the swap */
-		delta = ptoa(nobjsize - object->size);
+		/* Try to reserve additional swap space. */
+		delta = IDX_TO_OFF(nobjsize - object->size);
 		if (!swap_reserve_by_cred(delta, object->cred)) {
 			VM_OBJECT_WUNLOCK(object);
 			return (ENOMEM);