ipv6: introduce a hash table to store dst cache

Add a hash table into struct rt6_info in order to store dst caches
created by pmtu discovery and ip redirect in ipv6 routing code.
APIs to add dst cache, delete dst cache, find dst cache and update
dst cache in the hash table are implemented and will be used in later
commits.
This is a preparation work to move all cache routes into the exception
table instead of getting inserted into the fib6 tree.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_fib.h

@@ -98,6 +98,22 @@ struct rt6key {
 
 struct fib6_table;
 
+struct rt6_exception_bucket {
+	struct hlist_head	chain;
+	int			depth;
+};
+
+struct rt6_exception {
+	struct hlist_node	hlist;
+	struct rt6_info		*rt6i;
+	unsigned long		stamp;
+	struct rcu_head		rcu;
+};
+
+#define FIB6_EXCEPTION_BUCKET_SIZE_SHIFT 10
+#define FIB6_EXCEPTION_BUCKET_SIZE (1 << FIB6_EXCEPTION_BUCKET_SIZE_SHIFT)
+#define FIB6_MAX_DEPTH 5
+
 struct rt6_info {
 	struct dst_entry		dst;
 
@@ -134,12 +150,15 @@ struct rt6_info {
 
 	struct inet6_dev		*rt6i_idev;
 	struct rt6_info * __percpu	*rt6i_pcpu;
+	struct rt6_exception_bucket __rcu *rt6i_exception_bucket;
 
 	u32				rt6i_metric;
 	u32				rt6i_pmtu;
 	/* more non-fragment space at head required */
 	unsigned short			rt6i_nfheader_len;
 	u8				rt6i_protocol;
+	u8				exception_bucket_flushed:1,
+					unused:7;
 };
 
 static inline struct inet6_dev *ip6_dst_idev(struct dst_entry *dst)

include/net/ip6_route.h

@@ -95,6 +95,9 @@ int ip6_route_add(struct fib6_config *cfg, struct netlink_ext_ack *extack);
 int ip6_ins_rt(struct rt6_info *);
 int ip6_del_rt(struct rt6_info *);
 
+void rt6_flush_exceptions(struct rt6_info *rt);
+int rt6_remove_exception_rt(struct rt6_info *rt);
+
 static inline int ip6_route_get_saddr(struct net *net, struct rt6_info *rt,
 				      const struct in6_addr *daddr,
 				      unsigned int prefs,