From: Menglong Dong <imagedong@tencent.com>

Do the statistics of mptcp socket in use with sock_prot_inuse_add().
Therefore, we can get the count of used mptcp socket from
/proc/net/protocols:

& cat /proc/net/protocols
protocol  size sockets  memory press maxhdr  slab module     cl co di ac io in de sh ss gs se re sp bi br ha uh gp em
MPTCPv6   2048      0       0   no       0   yes  kernel      y  n  y  y  y  y  y  y  y  y  y  y  n  n  n  y  y  y  n
MPTCP     1896      1       0   no       0   yes  kernel      y  n  y  y  y  y  y  y  y  y  y  y  n  n  n  y  y  y  n

Signed-off-by: Menglong Dong <imagedong@tencent.com>
---
v6:
- introduce the 'MPTCP_INUSE' flag and check if msk is in use by it

v5:
- rebase to solve merge conflict

v4:
- rename MPTCP_DESTROIED to MPTCP_DESTROYED

v2:
- decrease the statistics for listening mptcp socket inuse with
  mptcp_listen_inuse_dec()
- add MPTCP_DESTROIED flags to store if mptcp_destroy_common() was
  called on the msk. For fallback case, we need to decrease the
  statistics only once, and mptcp_destroy_common() can be called
  more than once.
---
 net/mptcp/protocol.c |  8 +++++++-
 net/mptcp/protocol.h | 13 +++++++++++++
 net/mptcp/subflow.c  |  1 +
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 151d22c91b7e..0abdba83a576 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3068,6 +3068,7 @@ struct sock *mptcp_sk_clone(const struct sock *sk,
 	msk->snd_una = msk->write_seq;
 	msk->wnd_end = msk->snd_nxt + req->rsk_rcv_wnd;
 	msk->setsockopt_seq = mptcp_sk(sk)->setsockopt_seq;
+	clear_bit(MPTCP_INUSE, &msk->flags);
 	mptcp_init_sched(msk, mptcp_sk(sk)->sched);
 
 	if (mp_opt->suboptions & OPTIONS_MPTCP_MPC) {
@@ -3177,6 +3178,8 @@ void mptcp_destroy_common(struct mptcp_sock *msk, unsigned int flags)
 	skb_rbtree_purge(&msk->out_of_order_queue);
 	mptcp_data_unlock(sk);
 
+	mptcp_inuse_dec(sk);
+
 	/* move all the rx fwd alloc into the sk_mem_reclaim_final in
 	 * inet_sock_destruct() will dispose it
 	 */
@@ -3540,6 +3543,7 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 
 	mptcp_token_destroy(msk);
 	inet_sk_state_store(sk, TCP_SYN_SENT);
+	mptcp_inuse_inc(sk);
 	subflow = mptcp_subflow_ctx(ssock->sk);
 #ifdef CONFIG_TCP_MD5SIG
 	/* no MPTCP if MD5SIG is enabled on this socket or we may run out of
@@ -3672,8 +3676,10 @@ static int mptcp_listen(struct socket *sock, int backlog)
 
 	err = ssock->ops->listen(ssock, backlog);
 	inet_sk_state_store(sk, inet_sk_state_load(ssock->sk));
-	if (!err)
+	if (!err) {
+		mptcp_inuse_inc(sk);
 		mptcp_copy_inaddrs(sk, ssock->sk);
+	}
 
 unlock:
 	release_sock(sk);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 8f48f881adf8..d6d2ab17129b 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -116,6 +116,7 @@
 #define MPTCP_WORK_EOF		3
 #define MPTCP_FALLBACK_DONE	4
 #define MPTCP_WORK_CLOSE_SUBFLOW 5
+#define MPTCP_INUSE		6
 
 /* MPTCP socket release cb flags */
 #define MPTCP_PUSH_PENDING	1
@@ -383,6 +384,18 @@ static inline struct mptcp_data_frag *mptcp_rtx_head(const struct sock *sk)
 	return list_first_entry_or_null(&msk->rtx_queue, struct mptcp_data_frag, list);
 }
 
+static inline void mptcp_inuse_inc(const struct sock *sk)
+{
+	if (!test_and_set_bit(MPTCP_INUSE, &mptcp_sk(sk)->flags))
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
+}
+
+static inline void mptcp_inuse_dec(const struct sock *sk)
+{
+	if (test_and_clear_bit(MPTCP_INUSE, &mptcp_sk(sk)->flags))
+		sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
+}
+
 struct csum_pseudo_header {
 	__be64 data_seq;
 	__be32 subflow_seq;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 02a54d59697b..c5934140f3f3 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -749,6 +749,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 			mptcp_sk(new_msk)->setsockopt_seq = ctx->setsockopt_seq;
 			mptcp_pm_new_connection(mptcp_sk(new_msk), child, 1);
 			mptcp_token_accept(subflow_req, mptcp_sk(new_msk));
+			mptcp_inuse_inc(new_msk);
 			ctx->conn = new_msk;
 			new_msk = NULL;
 
-- 
2.37.2