net: Abstract default MTU metric calculation behind an accessor.

Like RTAX_ADVMSS, make the default calculation go through a dst_ops
method rather than caching the computation in the routing cache
entries.

Now dst metrics are pretty much left as-is when new entries are
created, thus optimizing metric sharing becomes a real possibility.

Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 20 insertions, 9 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 80997333db0..ae520963540 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -140,6 +140,7 @@ static unsigned long expires_ljiffies;
 
 static struct dst_entry *ipv4_dst_check(struct dst_entry *dst, u32 cookie);
 static unsigned int      ipv4_default_advmss(const struct dst_entry *dst);
+static unsigned int      ipv4_default_mtu(const struct dst_entry *dst);
 static void              ipv4_dst_destroy(struct dst_entry *dst);
 static struct dst_entry *ipv4_negative_advice(struct dst_entry *dst);
 static void              ipv4_link_failure(struct sk_buff *skb);
@@ -157,6 +158,7 @@ static struct dst_ops ipv4_dst_ops = {
         .gc =                   rt_garbage_collect,
         .check =                ipv4_dst_check,
         .default_advmss =       ipv4_default_advmss,
+        .default_mtu =          ipv4_default_mtu,
         .destroy =              ipv4_dst_destroy,
         .ifdown =               ipv4_dst_ifdown,
         .negative_advice =      ipv4_negative_advice,
@@ -1812,6 +1814,23 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
         return advmss;
 }
 
+static unsigned int ipv4_default_mtu(const struct dst_entry *dst)
+{
+        unsigned int mtu = dst->dev->mtu;
+
+        if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
+                const struct rtable *rt = (const struct rtable *) dst;
+
+                if (rt->rt_gateway != rt->rt_dst && mtu > 576)
+                        mtu = 576;
+        }
+
+        if (mtu > IP_MAX_MTU)
+                mtu = IP_MAX_MTU;
+
+        return mtu;
+}
+
 static void rt_set_nexthop(struct rtable *rt, struct fib_result *res, u32 itag)
 {
         struct dst_entry *dst = &rt->dst;
@@ -1822,18 +1841,10 @@ static void rt_set_nexthop(struct rtable *rt, struct fib_result *res, u32 itag)
                     FIB_RES_NH(*res).nh_scope == RT_SCOPE_LINK)
                         rt->rt_gateway = FIB_RES_GW(*res);
                 dst_import_metrics(dst, fi->fib_metrics);
-                if (fi->fib_mtu == 0) {
-                        dst_metric_set(dst, RTAX_MTU, dst->dev->mtu);
-                        if (dst_metric_locked(dst, RTAX_MTU) &&
-                            rt->rt_gateway != rt->rt_dst &&
-                            dst->dev->mtu > 576)
-                                dst_metric_set(dst, RTAX_MTU, 576);
-                }
 #ifdef CONFIG_NET_CLS_ROUTE
                 dst->tclassid = FIB_RES_NH(*res).nh_tclassid;
 #endif
-        } else
-                dst_metric_set(dst, RTAX_MTU, dst->dev->mtu);
+        }
 
         if (dst_mtu(dst) > IP_MAX_MTU)
                 dst_metric_set(dst, RTAX_MTU, IP_MAX_MTU);