If the initial subflow has been removed, we cannot know without checking
other counters, e.g. ss -ti <filter> | grep -c tcp-ulp-mptcp or
getsockopt(SOL_MPTCP, MPTCP_FULL_INFO, ...) (or others except MPTCP_INFO
of course) and then check mptcp_subflow_data->num_subflows to get the
total amount of subflows.

This patch adds a new counter mptcpi_subflows_total in mptcpi_flags to
store the total amount of subflows, including the initial one. A new
helper __mptcp_has_initial_subflow() is added to check whether the
initial subflow has been removed or not. With this helper, we can then
compute the total amount of subflows from mptcp_info by doing something
like:

    mptcpi_subflows_total = mptcpi_subflows +
            __mptcp_has_initial_subflow(msk).

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/428
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 include/uapi/linux/mptcp.h | 1 +
 net/mptcp/protocol.h       | 7 +++++++
 net/mptcp/sockopt.c        | 2 ++
 3 files changed, 10 insertions(+)

diff --git a/include/uapi/linux/mptcp.h b/include/uapi/linux/mptcp.h
index ee9c49f949a2..f89aaa67f10d 100644
--- a/include/uapi/linux/mptcp.h
+++ b/include/uapi/linux/mptcp.h
@@ -128,6 +128,7 @@ struct mptcp_info {
 	__u64	mptcpi_bytes_sent;
 	__u64	mptcpi_bytes_received;
 	__u64	mptcpi_bytes_acked;
+	__u8	mptcpi_subflows_total;
 };
 
 /*
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index fc041763dcb0..6ec1d967f22c 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1031,6 +1031,13 @@ static inline void __mptcp_do_fallback(struct mptcp_sock *msk)
 	set_bit(MPTCP_FALLBACK_DONE, &msk->flags);
 }
 
+static inline bool __mptcp_has_initial_subflow(const struct mptcp_sock *msk)
+{
+	struct sock *ssk = READ_ONCE(msk->first);
+
+	return ssk && inet_sk_state_load(ssk) != TCP_CLOSE;
+}
+
 static inline void mptcp_do_fallback(struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 1ae9fb6fabf1..d380575bbe69 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -934,6 +934,8 @@ void mptcp_diag_fill_info(struct mptcp_sock *msk, struct mptcp_info *info)
 	info->mptcpi_bytes_sent = msk->bytes_sent;
 	info->mptcpi_bytes_received = msk->bytes_received;
 	info->mptcpi_bytes_retrans = msk->bytes_retrans;
+	info->mptcpi_subflows_total = info->mptcpi_subflows +
+		__mptcp_has_initial_subflow(msk);
 	unlock_sock_fast(sk, slow);
 }
 EXPORT_SYMBOL_GPL(mptcp_diag_fill_info);
-- 
2.35.3