[PATCH v2 net] neighbor: Call __ipv4_neigh_lookup_noref in neigh_xmit

[David Ahern <dsahern@kernel.org>]

From: David Ahern <dsahern@gmail.com>

Commit cd9ff4de0107 changed the key for IFF_POINTOPOINT devices to
INADDR_ANY, but neigh_xmit which is used for MPLS encapsulations was not
updated to use the altered key. The result is that every packet Tx does
a lookup on the gateway address which does not find an entry, a new one
is created only to find the existing one in the table right before the
insert since arp_constructor was updated to reset the primary key. This
is seen in the allocs and destroys counters:
    ip -s -4 ntable show | head -10 | grep alloc

which increase for each packet showing the unnecessary overhread.

Fix by having neigh_xmit use __ipv4_neigh_lookup_noref for NEIGH_ARP_TABLE.
Define __ipv4_neigh_lookup_noref in case CONFIG_INET is not set.

v2
- define __ipv4_neigh_lookup_noref in case CONFIG_INET is not set as
  reported by kbuild test robot

Fixes: cd9ff4de0107 ("ipv4: Make neigh lookup keys for loopback/point-to-point devices be INADDR_ANY")
Reported-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: David Ahern <dsahern@gmail.com>

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 include/net/arp.h    | 8 ++++++++
 net/core/neighbour.c | 9 ++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/include/net/arp.h b/include/net/arp.h
index 977aabfcdc03..c8f580a0e6b1 100644
--- a/include/net/arp.h
+++ b/include/net/arp.h
@@ -18,6 +18,7 @@ static inline u32 arp_hashfn(const void *pkey, const struct net_device *dev, u32
 	return val * hash_rnd[0];
 }
 
+#ifdef CONFIG_INET
 static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
 {
 	if (dev->flags & (IFF_LOOPBACK | IFF_POINTOPOINT))
@@ -25,6 +26,13 @@ static inline struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev
 
 	return ___neigh_lookup_noref(&arp_tbl, neigh_key_eq32, arp_hashfn, &key, dev);
 }
+#else
+static inline
+struct neighbour *__ipv4_neigh_lookup_noref(struct net_device *dev, u32 key)
+{
+	return NULL;
+}
+#endif
 
 static inline struct neighbour *__ipv4_neigh_lookup(struct net_device *dev, u32 key)
 {
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 30f6fd8f68e0..0ba5018ccb7f 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -31,6 +31,7 @@
 #include <linux/times.h>
 #include <net/net_namespace.h>
 #include <net/neighbour.h>
+#include <net/arp.h>
 #include <net/dst.h>
 #include <net/sock.h>
 #include <net/netevent.h>
@@ -2982,7 +2983,13 @@ int neigh_xmit(int index, struct net_device *dev,
 		if (!tbl)
 			goto out;
 		rcu_read_lock_bh();
-		neigh = __neigh_lookup_noref(tbl, addr, dev);
+		if (index == NEIGH_ARP_TABLE) {
+			u32 key = *((u32 *)addr);
+
+			neigh = __ipv4_neigh_lookup_noref(dev, key);
+		} else {
+			neigh = __neigh_lookup_noref(tbl, addr, dev);
+		}
 		if (!neigh)
 			neigh = __neigh_create(tbl, addr, dev, false);
 		err = PTR_ERR(neigh);
-- 
2.11.0