tcp: Don't allocate tcp_death_row outside of struct netns_ipv4.

We will soon introduce an optional per-netns ehash and access hash
tables via net->ipv4.tcp_death_row->hashinfo instead of &tcp_hashinfo
in most places.

It could harm the fast path because dereferences of two fields in net
and tcp_death_row might incur two extra cache line misses.  To save one
dereference, let's place tcp_death_row back in netns_ipv4 and fetch
hashinfo via net->ipv4.tcp_death_row"."hashinfo.

Note tcp_death_row was initially placed in netns_ipv4, and commit
fbb82952 ("tcp: allocate tcp_death_row outside of struct netns_ipv4")
changed it to a pointer so that we can fire TIME_WAIT timers after freeing
net.  However, we don't do so after commit 04c494e6 ("Revert "tcp/dccp:
get rid of inet_twsk_purge()""), so we need not define tcp_death_row as a
pointer.

Also, we move refcount_dec_and_test(&tw_refcount) from tcp_sk_exit() to
tcp_sk_exit_batch() as a debug check.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/netns/ipv4.h

@@ -34,6 +34,7 @@ struct inet_hashinfo;
 struct inet_timewait_death_row {
 	refcount_t		tw_refcount;
 
+	/* Padding to avoid false sharing, tw_refcount can be often written */
 	struct inet_hashinfo 	*hashinfo ____cacheline_aligned_in_smp;
 	int			sysctl_max_tw_buckets;
 };
@@ -41,7 +42,7 @@ struct inet_timewait_death_row {
 struct tcp_fastopen_context;
 
 struct netns_ipv4 {
-	struct inet_timewait_death_row *tcp_death_row;
+	struct inet_timewait_death_row tcp_death_row;
 
 #ifdef CONFIG_SYSCTL
 	struct ctl_table_header	*forw_hdr;

net/ipv4/inet_timewait_sock.c

@@ -59,9 +59,7 @@ static void inet_twsk_kill(struct inet_timewait_sock *tw)
 	inet_twsk_bind_unhash(tw, hashinfo);
 	spin_unlock(&bhead->lock);
 
-	if (refcount_dec_and_test(&tw->tw_dr->tw_refcount))
-		kfree(tw->tw_dr);
-
+	refcount_dec(&tw->tw_dr->tw_refcount);
 	inet_twsk_put(tw);
 }
 

net/ipv4/proc.c

@@ -59,7 +59,7 @@ static int sockstat_seq_show(struct seq_file *seq, void *v)
 	socket_seq_show(seq);
 	seq_printf(seq, "TCP: inuse %d orphan %d tw %d alloc %d mem %ld\n",
 		   sock_prot_inuse_get(net, &tcp_prot), orphans,
-		   refcount_read(&net->ipv4.tcp_death_row->tw_refcount) - 1,
+		   refcount_read(&net->ipv4.tcp_death_row.tw_refcount) - 1,
 		   sockets, proto_memory_allocated(&tcp_prot));
 	seq_printf(seq, "UDP: inuse %d mem %ld\n",
 		   sock_prot_inuse_get(net, &udp_prot),

net/ipv4/sysctl_net_ipv4.c

@@ -530,10 +530,9 @@ static struct ctl_table ipv4_table[] = {
 };
 
 static struct ctl_table ipv4_net_table[] = {
-	/* tcp_max_tw_buckets must be first in this table. */
 	{
 		.procname	= "tcp_max_tw_buckets",
-/*		.data		= &init_net.ipv4.tcp_death_row.sysctl_max_tw_buckets, */
+		.data		= &init_net.ipv4.tcp_death_row.sysctl_max_tw_buckets,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec
@@ -1361,8 +1360,7 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
 		if (!table)
 			goto err_alloc;
 
-		/* skip first entry (sysctl_max_tw_buckets) */
-		for (i = 1; i < ARRAY_SIZE(ipv4_net_table) - 1; i++) {
+		for (i = 0; i < ARRAY_SIZE(ipv4_net_table) - 1; i++) {
 			if (table[i].data) {
 				/* Update the variables to point into
 				 * the current struct net
@@ -1377,8 +1375,6 @@ static __net_init int ipv4_sysctl_init_net(struct net *net)
 		}
 	}
 
-	table[0].data = &net->ipv4.tcp_death_row->sysctl_max_tw_buckets;
-
 	net->ipv4.ipv4_hdr = register_net_sysctl(net, "net/ipv4", table);
 	if (!net->ipv4.ipv4_hdr)
 		goto err_reg;

net/ipv4/tcp_ipv4.c

@@ -292,7 +292,7 @@ int tcp_v4_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	 * complete initialization after this.
 	 */
 	tcp_set_state(sk, TCP_SYN_SENT);
-	tcp_death_row = net->ipv4.tcp_death_row;
+	tcp_death_row = &net->ipv4.tcp_death_row;
 	err = inet_hash_connect(tcp_death_row, sk);
 	if (err)
 		goto failure;
@@ -3091,13 +3091,9 @@ EXPORT_SYMBOL(tcp_prot);
 
 static void __net_exit tcp_sk_exit(struct net *net)
 {
-	struct inet_timewait_death_row *tcp_death_row = net->ipv4.tcp_death_row;
-
 	if (net->ipv4.tcp_congestion_control)
 		bpf_module_put(net->ipv4.tcp_congestion_control,
 			       net->ipv4.tcp_congestion_control->owner);
-	if (refcount_dec_and_test(&tcp_death_row->tw_refcount))
-		kfree(tcp_death_row);
 }
 
 static int __net_init tcp_sk_init(struct net *net)
@@ -3129,13 +3125,10 @@ static int __net_init tcp_sk_init(struct net *net)
 	net->ipv4.sysctl_tcp_tw_reuse = 2;
 	net->ipv4.sysctl_tcp_no_ssthresh_metrics_save = 1;
 
-	net->ipv4.tcp_death_row = kzalloc(sizeof(struct inet_timewait_death_row), GFP_KERNEL);
-	if (!net->ipv4.tcp_death_row)
-		return -ENOMEM;
-	refcount_set(&net->ipv4.tcp_death_row->tw_refcount, 1);
+	refcount_set(&net->ipv4.tcp_death_row.tw_refcount, 1);
 	cnt = tcp_hashinfo.ehash_mask + 1;
-	net->ipv4.tcp_death_row->sysctl_max_tw_buckets = cnt / 2;
-	net->ipv4.tcp_death_row->hashinfo = &tcp_hashinfo;
+	net->ipv4.tcp_death_row.sysctl_max_tw_buckets = cnt / 2;
+	net->ipv4.tcp_death_row.hashinfo = &tcp_hashinfo;
 
 	net->ipv4.sysctl_max_syn_backlog = max(128, cnt / 128);
 	net->ipv4.sysctl_tcp_sack = 1;
@@ -3201,8 +3194,10 @@ static void __net_exit tcp_sk_exit_batch(struct list_head *net_exit_list)
 
 	inet_twsk_purge(&tcp_hashinfo, AF_INET);
 
-	list_for_each_entry(net, net_exit_list, exit_list)
+	list_for_each_entry(net, net_exit_list, exit_list) {
+		WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
 		tcp_fastopen_ctx_destroy(net);
+	}
 }
 
 static struct pernet_operations __net_initdata tcp_sk_ops = {

net/ipv4/tcp_minisocks.c

@@ -250,7 +250,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo)
 	struct net *net = sock_net(sk);
 	struct inet_timewait_sock *tw;
 
-	tw = inet_twsk_alloc(sk, net->ipv4.tcp_death_row, state);
+	tw = inet_twsk_alloc(sk, &net->ipv4.tcp_death_row, state);
 
 	if (tw) {
 		struct tcp_timewait_sock *tcptw = tcp_twsk((struct sock *)tw);

net/ipv6/tcp_ipv6.c

@@ -325,7 +325,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr *uaddr,
 	inet->inet_dport = usin->sin6_port;
 
 	tcp_set_state(sk, TCP_SYN_SENT);
-	tcp_death_row = net->ipv4.tcp_death_row;
+	tcp_death_row = &net->ipv4.tcp_death_row;
 	err = inet6_hash_connect(tcp_death_row, sk);
 	if (err)
 		goto late_failure;