batman-adv: Check hardif MTU against runtime MTU

If the MTU of the soft/mesh interface was already reduced (enough), it is
not necessary to print a warning about a hard interface not having a MTU to
transport ethernet payloads of 1500 bytes.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/hard-interface.c

@@ -9,6 +9,7 @@
 
 #include <linux/atomic.h>
 #include <linux/byteorder/generic.h>
+#include <linux/compiler.h>
 #include <linux/container_of.h>
 #include <linux/errno.h>
 #include <linux/gfp.h>
@@ -699,9 +700,14 @@ int batadv_hardif_enable_interface(struct batadv_hard_iface *hard_iface,
 	struct batadv_priv *bat_priv;
 	__be16 ethertype = htons(ETH_P_BATMAN);
 	int max_header_len = batadv_max_header_len();
+	unsigned int required_mtu;
+	unsigned int hardif_mtu;
 	int ret;
 
-	if (hard_iface->net_dev->mtu < ETH_MIN_MTU + max_header_len)
+	hardif_mtu = READ_ONCE(hard_iface->net_dev->mtu);
+	required_mtu = READ_ONCE(soft_iface->mtu) + max_header_len;
+
+	if (hardif_mtu < ETH_MIN_MTU + max_header_len)
 		return -EINVAL;
 
 	if (hard_iface->if_status != BATADV_IF_NOT_IN_USE)
@@ -734,18 +740,18 @@ int batadv_hardif_enable_interface(struct batadv_hard_iface *hard_iface,
 		    hard_iface->net_dev->name);
 
 	if (atomic_read(&bat_priv->fragmentation) &&
-	    hard_iface->net_dev->mtu < ETH_DATA_LEN + max_header_len)
+	    hardif_mtu < required_mtu)
 		batadv_info(hard_iface->soft_iface,
 			    "The MTU of interface %s is too small (%i) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to %i would solve the problem.\n",
-			    hard_iface->net_dev->name, hard_iface->net_dev->mtu,
-			    ETH_DATA_LEN + max_header_len);
+			    hard_iface->net_dev->name, hardif_mtu,
+			    required_mtu);
 
 	if (!atomic_read(&bat_priv->fragmentation) &&
-	    hard_iface->net_dev->mtu < ETH_DATA_LEN + max_header_len)
+	    hardif_mtu < required_mtu)
 		batadv_info(hard_iface->soft_iface,
 			    "The MTU of interface %s is too small (%i) to handle the transport of batman-adv packets. If you experience problems getting traffic through try increasing the MTU to %i.\n",
-			    hard_iface->net_dev->name, hard_iface->net_dev->mtu,
-			    ETH_DATA_LEN + max_header_len);
+			    hard_iface->net_dev->name, hardif_mtu,
+			    required_mtu);
 
 	if (batadv_hardif_is_iface_up(hard_iface))
 		batadv_hardif_activate_interface(hard_iface);