sendmsg return EINVAL caused by ARP table full

int udp_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
		size_t len) ///len: msg total length
{
…
	/* Lockless fast path for the non-corking case. */
	if (!corkreq) { ///no cork
		skb = ip_make_skb(sk, fl4, getfrag, msg->msg_iov, ulen,
				  sizeof(struct udphdr), &ipc, &rt,
				  msg->msg_flags);
		err = PTR_ERR(skb);
		if (!IS_ERR_OR_NULL(skb))
			err = udp_send_skb(skb, fl4);
		goto out;
	}

static int ip_finish_output(struct sk_buff *skb)
{
#if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)
	/* Policy lookup after SNAT yielded a new policy */
	if (skb_dst(skb)->xfrm != NULL) {
		IPCB(skb)->flags |= IPSKB_REROUTED;
		return dst_output(skb);
	}
#endif
	if (skb->len > ip_skb_dst_mtu(skb) && !skb_is_gso(skb))///for UDP, if no UDP, skb_shinfo(skb)->gso_size will be zero
		return ip_fragment(skb, ip_finish_output2);
	else
		return ip_finish_output2(skb);
}

static inline int ip_finish_output2(struct sk_buff *skb)
{

…
	rcu_read_lock_bh();
	nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr);//ex: gateway L3 address or dest IP
	neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
	if (unlikely(!neigh)) ///create L3->L2 entry
		neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);
	if (!IS_ERR(neigh)) {
		int res = dst_neigh_output(dst, neigh, skb);

		rcu_read_unlock_bh();
		return res;
	}
	rcu_read_unlock_bh();

	net_dbg_ratelimited("%s: No header cache and no neighbour!\n",
			    __func__);
	kfree_skb(skb);
	return -EINVAL;
}

struct neighbour *__neigh_create(struct neigh_table *tbl, const void *pkey,
				 struct net_device *dev, bool want_ref)
{
	u32 hash_val;
	int key_len = tbl->key_len;
	int error;
	struct neighbour *n1, *rc, *n = neigh_alloc(tbl, dev);
	struct neigh_hash_table *nht;

	if (!n) {
		rc = ERR_PTR(-ENOBUFS);
		goto out;
	}
…

static inline int ip_finish_output2(struct sk_buff *skb)
{
	net_dbg_ratelimited("%s: No header cache and no neighbour!\n",
			    __func__);
	kfree_skb(skb);
	return -EINVAL;
}

#define net_dbg_ratelimited(fmt, ...)				\
	net_ratelimited_function(pr_debug, fmt, ##__VA_ARGS__)

static inline int ip_finish_output2(struct sk_buff *skb)
{
	nexthop = (__force u32) rt_nexthop(rt, ip_hdr(skb)->daddr);//ex: gateway L3 address or dest IP
	neigh = __ipv4_neigh_lookup_noref(dev, nexthop);
	if (unlikely(!neigh)) ///create L3->L2 entry
		neigh = __neigh_create(&arp_tbl, &nexthop, dev, false);
…

static inline __be32 rt_nexthop(const struct rtable *rt, __be32 daddr)
{
	if (rt->rt_gateway) ///return gateway IP
		return rt->rt_gateway;
	return daddr; /// return dest IP
}

static int __net_init arp_net_init(struct net *net)
{
	if (!proc_create("arp", S_IRUGO, net->proc_net, &arp_seq_fops)) ///proc/net/arp
		return -ENOMEM;
	return 0;
}

static struct neighbour *neigh_get_next(struct seq_file *seq,
					struct neighbour *n,
					loff_t *pos)
{
	struct neigh_seq_state *state = seq->private;
	struct net *net = seq_file_net(seq);
	struct neigh_hash_table *nht = state->nht;
…
	while (1) {
		while (n) {
			if (!net_eq(dev_net(n->dev), net)) ///check net ns
				goto next;
			if (state->neigh_sub_iter) {
				void *v = state->neigh_sub_iter(state, n, pos);
				if (v)
					return n;
				goto next;
			}
			if (!(state->flags & NEIGH_SEQ_SKIP_NOARP)) ///skip NOARP
				break;

			if (n->nud_state & ~NUD_NOARP) ///dev is set NOARP
				break;
next:
			n = rcu_dereference_bh(n->next);
		}

static void neigh_table_init_no_netlink(struct neigh_table *tbl)
{
	tbl->stats = alloc_percpu(struct neigh_statistics);
	if (!tbl->stats)
		panic("cannot create neighbour cache statistics");

#ifdef CONFIG_PROC_FS
	if (!proc_create_data(tbl->id, 0, init_net.proc_net_stat, ///proc/net/stat/arp_cache, only exsit in init.net
			      &neigh_stat_seq_fops, tbl))
		panic("cannot create neighbour proc dir entry");
#endif



static int neigh_stat_seq_show(struct seq_file *seq, void *v)
{
	struct neigh_table *tbl = seq->private;
	struct neigh_statistics *st = v;

	if (v == SEQ_START_TOKEN) {
		seq_printf(seq, "entries  allocs destroys hash_grows  lookups hits  res_failed  rcv_probes_mcast rcv_probes_ucast  periodic_gc_runs forced_gc_runs unresolved_discards\n");
		return 0;
	}

	seq_printf(seq, "%08x  %08lx %08lx %08lx  %08lx %08lx  %08lx  "
			"%08lx %08lx  %08lx %08lx %08lx\n",
		   atomic_read(&tbl->entries), ///ARP表项数量

		   st->allocs,
		   st->destroys,
		   st->hash_grows,

		   st->lookups,
		   st->hits,

		   st->res_failed,

		   st->rcv_probes_mcast,
		   st->rcv_probes_ucast,

		   st->periodic_gc_runs,
		   st->forced_gc_runs,
		   st->unres_discards
		   );

	return 0;
}