MFC r339909: Allow changing lagg(4) MTU.

Previously, changing the MTU would require destroying the lagg and
creating a new one. Now it is allowed to change the MTU of
the lagg interface and the MTU of the ports will be set to match.

If any port cannot set the new MTU, all ports are reverted to the original
MTU of the lagg. Additionally, when adding ports, the MTU of a port will be
automatically set to the MTU of the lagg. As always, the MTU of the lagg is
initially determined by the MTU of the first port added. If adding an
interface as a port for some reason fails, that interface is reverted to its
original MTU.

Submitted by:	Ryan Moeller <ryan@freqlabs.com>
Relnotes:	Yes
Sponsored by:	iXsystems Inc.

sys/net/if_lagg.c

@@ -633,11 +633,18 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 {
 	struct lagg_softc *sc_ptr;
 	struct lagg_port *lp, *tlp;
-	int error, i;
+	struct ifreq ifr;
+	int error, i, oldmtu;
 	uint64_t *pval;
 
 	LAGG_XLOCK_ASSERT(sc);
 
+	if (sc->sc_ifp == ifp) {
+		if_printf(sc->sc_ifp,
+		    "cannot add a lagg to itself as a port\n");
+		return (EINVAL);
+	}
+
 	/* Limit the maximal number of lagg ports */
 	if (sc->sc_count >= LAGG_MAX_PORTS)
 		return (ENOSPC);
@@ -656,12 +663,25 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 		return (EPROTONOSUPPORT);
 
 	/* Allow the first Ethernet member to define the MTU */
-	if (CK_SLIST_EMPTY(&sc->sc_ports))
+	oldmtu = -1;
+	if (CK_SLIST_EMPTY(&sc->sc_ports)) {
 		sc->sc_ifp->if_mtu = ifp->if_mtu;
-	else if (sc->sc_ifp->if_mtu != ifp->if_mtu) {
-		if_printf(sc->sc_ifp, "invalid MTU for %s\n",
-		    ifp->if_xname);
-		return (EINVAL);
+	} else if (sc->sc_ifp->if_mtu != ifp->if_mtu) {
+		if (ifp->if_ioctl == NULL) {
+			if_printf(sc->sc_ifp, "cannot change MTU for %s\n",
+			    ifp->if_xname);
+			return (EINVAL);
+		}
+		oldmtu = ifp->if_mtu;
+		strlcpy(ifr.ifr_name, ifp->if_xname, sizeof(ifr.ifr_name));
+		ifr.ifr_mtu = sc->sc_ifp->if_mtu;
+		error = (*ifp->if_ioctl)(ifp, SIOCSIFMTU, (caddr_t)&ifr);
+		if (error != 0) {
+			if_printf(sc->sc_ifp, "invalid MTU for %s\n",
+			    ifp->if_xname);
+			return (error);
+		}
+		ifr.ifr_mtu = oldmtu;
 	}
 
 	lp = malloc(sizeof(struct lagg_port), M_DEVBUF, M_WAITOK|M_ZERO);
@@ -673,6 +693,9 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 		if (ifp == sc_ptr->sc_ifp) {
 			LAGG_LIST_UNLOCK();
 			free(lp, M_DEVBUF);
+			if (oldmtu != -1)
+				(*ifp->if_ioctl)(ifp, SIOCSIFMTU,
+				    (caddr_t)&ifr);
 			return (EINVAL);
 			/* XXX disable stacking for the moment, its untested */
 #ifdef LAGG_PORT_STACKING
@@ -681,6 +704,9 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 			    LAGG_MAX_STACKING) {
 				LAGG_LIST_UNLOCK();
 				free(lp, M_DEVBUF);
+				if (oldmtu != -1)
+					(*ifp->if_ioctl)(ifp, SIOCSIFMTU,
+					    (caddr_t)&ifr);
 				return (E2BIG);
 			}
 #endif
@@ -746,6 +772,8 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 	if ((error = lagg_proto_addport(sc, lp)) != 0) {
 		/* Remove the port, without calling pr_delport. */
 		lagg_port_destroy(lp, 0);
+		if (oldmtu != -1)
+			(*ifp->if_ioctl)(ifp, SIOCSIFMTU, (caddr_t)&ifr);
 		return (error);
 	}
 
@@ -1464,8 +1492,31 @@ lagg_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
 		break;
 
 	case SIOCSIFMTU:
-		/* Do not allow the MTU to be directly changed */
-		error = EINVAL;
+		LAGG_XLOCK(sc);
+		CK_SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+			if (lp->lp_ioctl != NULL)
+				error = (*lp->lp_ioctl)(lp->lp_ifp, cmd, data);
+			else
+				error = EINVAL;
+			if (error != 0) {
+				if_printf(ifp,
+				    "failed to change MTU to %d on port %s, "
+				    "reverting all ports to original MTU (%d)\n",
+				    ifr->ifr_mtu, lp->lp_ifp->if_xname, ifp->if_mtu);
+				break;
+			}
+		}
+		if (error == 0) {
+			ifp->if_mtu = ifr->ifr_mtu;
+		} else {
+			/* set every port back to the original MTU */
+			ifr->ifr_mtu = ifp->if_mtu;
+			CK_SLIST_FOREACH(lp, &sc->sc_ports, lp_entries) {
+				if (lp->lp_ioctl != NULL)
+					(*lp->lp_ioctl)(lp->lp_ifp, cmd, data);
+			}
+		}
+		LAGG_XUNLOCK(sc);
 		break;
 
 	default: