net: bridge: disable bridge MTU auto tuning if it was set manually

As Roopa noted today the biggest source of problems when configuring
bridge and ports is that the bridge MTU keeps changing automatically on
port events (add/del/changemtu). That leads to inconsistent behaviour
and network config software needs to chase the MTU and fix it on each
such event. Let's improve on that situation and allow for the user to
set any MTU within ETH_MIN/MAX limits, but once manually configured it
is the user's responsibility to keep it correct afterwards.

In case the MTU isn't manually set - the behaviour reverts to the
previous and the bridge follows the minimum MTU.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/bridge/br.c b/net/bridge/br.c
index 565ff055813b30bc87daaed0815e0b7e47f2eb6f..671d13c10f6f4ed8118fe31fcceb28d7cde6edf1 100644 (file)
--- a/net/bridge/br.c
+++ b/net/bridge/br.c
@@ -52,7 +52,7 @@ static int br_device_event(struct notifier_block *unused, unsigned long event, v
 
        switch (event) {
        case NETDEV_CHANGEMTU:
-               dev_set_mtu(br->dev, br_mtu(br, false));
+               br_mtu_auto_adjust(br);
                break;
 
        case NETDEV_CHANGEADDR:

diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index edb9967eb165c987e55bc822ad9a057fab0483b9..e682a668ce57c9c0f5b4db62438a640f03d65b34 100644 (file)
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -225,11 +225,10 @@ static int br_change_mtu(struct net_device *dev, int new_mtu)
 {
        struct net_bridge *br = netdev_priv(dev);
 
-       if (new_mtu > br_mtu(br, br_vlan_enabled(dev)))
-               return -EINVAL;
-
        dev->mtu = new_mtu;
 
+       /* this flag will be cleared if the MTU was automatically adjusted */
+       br->mtu_set_by_user = true;
 #if IS_ENABLED(CONFIG_BRIDGE_NETFILTER)
        /* remember the MTU in the rtable for PMTU */
        dst_metric_set(&br->fake_rtable.dst, RTAX_MTU, new_mtu);

diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
index 7d5dc5a91084034c6f91de6722c29297caa043c8..82c1a6f430b37d8fb9b4713ad7ab370a3ef5b990 100644 (file)
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -424,28 +424,34 @@ int br_del_bridge(struct net *net, const char *name)
        return ret;
 }
 
-/* MTU of the bridge pseudo-device: ETH_DATA_LEN if there are no ports, the
- * minimum of the ports if @max is false or the maximum if it's true
- */
-int br_mtu(const struct net_bridge *br, bool max)
+/* MTU of the bridge pseudo-device: ETH_DATA_LEN or the minimum of the ports */
+static int br_mtu_min(const struct net_bridge *br)
 {
        const struct net_bridge_port *p;
        int ret_mtu = 0;
 
-       ASSERT_RTNL();
-
-       list_for_each_entry(p, &br->port_list, list) {
-               if (!max) {
-                       if (!ret_mtu || ret_mtu > p->dev->mtu)
-                               ret_mtu = p->dev->mtu;
-               } else if (p->dev->mtu > ret_mtu) {
+       list_for_each_entry(p, &br->port_list, list)
+               if (!ret_mtu || ret_mtu > p->dev->mtu)
                        ret_mtu = p->dev->mtu;
-               }
-       }
 
        return ret_mtu ? ret_mtu : ETH_DATA_LEN;
 }
 
+void br_mtu_auto_adjust(struct net_bridge *br)
+{
+       ASSERT_RTNL();
+
+       /* if the bridge MTU was manually configured don't mess with it */
+       if (br->mtu_set_by_user)
+               return;
+
+       /* change to the minimum MTU and clear the flag which was set by
+        * the bridge ndo_change_mtu callback
+        */
+       dev_set_mtu(br->dev, br_mtu_min(br));
+       br->mtu_set_by_user = false;
+}
+
 static void br_set_gso_limits(struct net_bridge *br)
 {
        unsigned int gso_max_size = GSO_MAX_SIZE;
@@ -597,7 +603,7 @@ int br_add_if(struct net_bridge *br, struct net_device *dev,
        if (changed_addr)
                call_netdevice_notifiers(NETDEV_CHANGEADDR, br->dev);
 
-       dev_set_mtu(br->dev, br_mtu(br, false));
+       br_mtu_auto_adjust(br);
        br_set_gso_limits(br);
 
        kobject_uevent(&p->kobj, KOBJ_ADD);
@@ -644,7 +650,7 @@ int br_del_if(struct net_bridge *br, struct net_device *dev)
         */
        del_nbp(p);
 
-       dev_set_mtu(br->dev, br_mtu(br, false));
+       br_mtu_auto_adjust(br);
        br_set_gso_limits(br);
 
        spin_lock_bh(&br->lock);

diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index 586f84b9670d97402bf0a60efbb144224b83ce01..a7cb3ece5031092f9df529864affdfafdfd1cf28 100644 (file)
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -410,6 +410,7 @@ struct net_bridge {
        int offload_fwd_mark;
 #endif
        bool                            neigh_suppress_enabled;
+       bool                            mtu_set_by_user;
        struct hlist_head               fdb_list;
 };
 
@@ -578,7 +579,7 @@ int br_del_bridge(struct net *net, const char *name);
 int br_add_if(struct net_bridge *br, struct net_device *dev,
              struct netlink_ext_ack *extack);
 int br_del_if(struct net_bridge *br, struct net_device *dev);
-int br_mtu(const struct net_bridge *br, bool max);
+void br_mtu_auto_adjust(struct net_bridge *br);
 netdev_features_t br_features_recompute(struct net_bridge *br,
                                        netdev_features_t features);
 void br_port_flags_change(struct net_bridge_port *port, unsigned long mask);