[PATCH net-next 0/2] mptcp: track more fallback cases

[PATCH net-next 0/2] mptcp: track more fallback cases

[Matthieu Baerts (NGI0)]

This series has two patches linked to fallback to TCP:

- Patch 1: additional MIB counters for remaining error code paths around
  fallback

- Patch 2: remove dedicated pr_debug() linked to fallback now that
  everything should be covered by dedicated MIB counters.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Paolo Abeni (2):
      mptcp: track fallbacks accurately via mibs
      mptcp: remove pr_fallback()

 net/mptcp/ctrl.c     |  4 ++--
 net/mptcp/mib.c      |  5 +++++
 net/mptcp/mib.h      |  7 +++++++
 net/mptcp/options.c  |  5 +++--
 net/mptcp/protocol.c | 45 ++++++++++++++++++++++++++++++---------------
 net/mptcp/protocol.h | 34 ++++++++--------------------------
 net/mptcp/subflow.c  | 16 +++++++---------
 7 files changed, 62 insertions(+), 54 deletions(-)
---
base-commit: 56613001dfc9b2e35e2d6ba857cbc2eb0bac4272
change-id: 20250722-net-next-mptcp-track-fallbacks-4fd69ed72d4e

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH net-next 1/2] mptcp: track fallbacks accurately via mibs

[Matthieu Baerts (NGI0)]

From: Paolo Abeni <pabeni@redhat.com>

Add the mibs required to cover the few possible fallback causes still
lacking suck info.

Move the relevant mib increment into the fallback helper, so that no
eventual future fallback operation will miss a paired mib increment.

Additionally track failed fallback via its own mib, such mib is
incremented only when a fallback mandated by the protocol fails - due to
racing subflow creation.

While at the above, rename an existing helper to reduce long lines
problems all along.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/ctrl.c     |  4 ++--
 net/mptcp/mib.c      |  5 +++++
 net/mptcp/mib.h      |  7 +++++++
 net/mptcp/options.c  |  4 +++-
 net/mptcp/protocol.c | 44 ++++++++++++++++++++++++++++++--------------
 net/mptcp/protocol.h | 31 ++++++++-----------------------
 net/mptcp/subflow.c  | 12 +++++++-----
 7 files changed, 62 insertions(+), 45 deletions(-)

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index d9290c5bb6c7956ca98319259f92b812680f74f7..fed40dae5583a35914da2cb1c52f37830d72705e 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -533,9 +533,9 @@ void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
 	to_max = mptcp_get_pernet(net)->syn_retrans_before_tcp_fallback;
 
 	if (timeouts == to_max || (timeouts < to_max && expired)) {
-		MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
 		subflow->mpc_drop = 1;
-		mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
+		mptcp_early_fallback(mptcp_sk(subflow->conn), subflow,
+				     MPTCP_MIB_MPCAPABLEACTIVEDROP);
 	}
 }
 
diff --git a/net/mptcp/mib.c b/net/mptcp/mib.c
index 0c24545f0e8df95b3475bfccc7a2f2ce440f7ad2..cf879c188ca26033ff2bde458fdb8f8fb0d87769 100644
--- a/net/mptcp/mib.c
+++ b/net/mptcp/mib.c
@@ -80,6 +80,11 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("RcvWndConflict", MPTCP_MIB_RCVWNDCONFLICT),
 	SNMP_MIB_ITEM("MPCurrEstab", MPTCP_MIB_CURRESTAB),
 	SNMP_MIB_ITEM("Blackhole", MPTCP_MIB_BLACKHOLE),
+	SNMP_MIB_ITEM("MPCapableDataFallback", MPTCP_MIB_MPCAPABLEDATAFALLBACK),
+	SNMP_MIB_ITEM("MD5SigFallback", MPTCP_MIB_MD5SIGFALLBACK),
+	SNMP_MIB_ITEM("DssFallback", MPTCP_MIB_DSSFALLBACK),
+	SNMP_MIB_ITEM("SimultConnectFallback", MPTCP_MIB_SIMULTCONNFALLBACK),
+	SNMP_MIB_ITEM("FallbackFailed", MPTCP_MIB_FALLBACKFAILED),
 	SNMP_MIB_SENTINEL
 };
 
diff --git a/net/mptcp/mib.h b/net/mptcp/mib.h
index 250c6b77977e8f846b5741304f7841a922f51967..309bac6fea3252ae395bcfb159917cdccc35277a 100644
--- a/net/mptcp/mib.h
+++ b/net/mptcp/mib.h
@@ -81,6 +81,13 @@ enum linux_mptcp_mib_field {
 	MPTCP_MIB_RCVWNDCONFLICT,	/* Conflict with while updating msk rcv wnd */
 	MPTCP_MIB_CURRESTAB,		/* Current established MPTCP connections */
 	MPTCP_MIB_BLACKHOLE,		/* A blackhole has been detected */
+	MPTCP_MIB_MPCAPABLEDATAFALLBACK,	/* Missing DSS/MPC+data on first
+					 * established packet
+					 */
+	MPTCP_MIB_MD5SIGFALLBACK,	/* Conflicting TCP option enabled */
+	MPTCP_MIB_DSSFALLBACK,		/* Bad or missing DSS */
+	MPTCP_MIB_SIMULTCONNFALLBACK,	/* Simultaneous connect */
+	MPTCP_MIB_FALLBACKFAILED,	/* Can't fallback due to msk status */
 	__MPTCP_MIB_MAX
 };
 
diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 1f898888b22357be395a2fd3e585df2375c8b2d2..6cf02344249a65d00b5832ebb69dd39d6e5fe98e 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -978,8 +978,10 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 		if (subflow->mp_join)
 			goto reset;
 		subflow->mp_capable = 0;
-		if (!mptcp_try_fallback(ssk))
+		if (!mptcp_try_fallback(ssk, MPTCP_MIB_MPCAPABLEDATAFALLBACK)) {
+			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_FALLBACKFAILED);
 			goto reset;
+		}
 		pr_fallback(msk);
 		return false;
 	}
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 6c448a0be9495b22ced4a2b51da2f80831040aba..88bf092f230abbecddc965b4242f711a5ba9d6b6 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -68,6 +68,26 @@ static const struct proto_ops *mptcp_fallback_tcp_ops(const struct sock *sk)
 	return &inet_stream_ops;
 }
 
+bool __mptcp_try_fallback(struct mptcp_sock *msk, int fb_mib)
+{
+	struct net *net = sock_net((struct sock *)msk);
+
+	if (__mptcp_check_fallback(msk))
+		return true;
+
+	spin_lock_bh(&msk->fallback_lock);
+	if (!msk->allow_infinite_fallback) {
+		spin_unlock_bh(&msk->fallback_lock);
+		return false;
+	}
+
+	msk->allow_subflows = false;
+	set_bit(MPTCP_FALLBACK_DONE, &msk->flags);
+	__MPTCP_INC_STATS(net, fb_mib);
+	spin_unlock_bh(&msk->fallback_lock);
+	return true;
+}
+
 static int __mptcp_socket_create(struct mptcp_sock *msk)
 {
 	struct mptcp_subflow_context *subflow;
@@ -561,10 +581,7 @@ static bool mptcp_check_data_fin(struct sock *sk)
 
 static void mptcp_dss_corruption(struct mptcp_sock *msk, struct sock *ssk)
 {
-	if (mptcp_try_fallback(ssk)) {
-		MPTCP_INC_STATS(sock_net(ssk),
-				MPTCP_MIB_DSSCORRUPTIONFALLBACK);
-	} else {
+	if (!mptcp_try_fallback(ssk, MPTCP_MIB_DSSCORRUPTIONFALLBACK)) {
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSCORRUPTIONRESET);
 		mptcp_subflow_reset(ssk);
 	}
@@ -1143,12 +1160,12 @@ static void mptcp_update_infinite_map(struct mptcp_sock *msk,
 	mpext->infinite_map = 1;
 	mpext->data_len = 0;
 
-	if (!mptcp_try_fallback(ssk)) {
+	if (!mptcp_try_fallback(ssk, MPTCP_MIB_INFINITEMAPTX)) {
+		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_FALLBACKFAILED);
 		mptcp_subflow_reset(ssk);
 		return;
 	}
 
-	MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_INFINITEMAPTX);
 	mptcp_subflow_ctx(ssk)->send_infinite_map = 0;
 	pr_fallback(msk);
 }
@@ -3689,16 +3706,15 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	 * TCP option space.
 	 */
 	if (rcu_access_pointer(tcp_sk(ssk)->md5sig_info))
-		mptcp_subflow_early_fallback(msk, subflow);
+		mptcp_early_fallback(msk, subflow, MPTCP_MIB_MD5SIGFALLBACK);
 #endif
 	if (subflow->request_mptcp) {
-		if (mptcp_active_should_disable(sk)) {
-			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDISABLED);
-			mptcp_subflow_early_fallback(msk, subflow);
-		} else if (mptcp_token_new_connect(ssk) < 0) {
-			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_TOKENFALLBACKINIT);
-			mptcp_subflow_early_fallback(msk, subflow);
-		}
+		if (mptcp_active_should_disable(sk))
+			mptcp_early_fallback(msk, subflow,
+					     MPTCP_MIB_MPCAPABLEACTIVEDISABLED);
+		else if (mptcp_token_new_connect(ssk) < 0)
+			mptcp_early_fallback(msk, subflow,
+					     MPTCP_MIB_TOKENFALLBACKINIT);
 	}
 
 	WRITE_ONCE(msk->write_seq, subflow->idsn);
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 1a32edf6f34364eedd5d077eae7a82a7db8c3a9e..912f048994a196304a80cde90747d4a512fb3d3d 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1223,24 +1223,6 @@ static inline bool mptcp_check_fallback(const struct sock *sk)
 	return __mptcp_check_fallback(msk);
 }
 
-static inline bool __mptcp_try_fallback(struct mptcp_sock *msk)
-{
-	if (__mptcp_check_fallback(msk)) {
-		pr_debug("TCP fallback already done (msk=%p)\n", msk);
-		return true;
-	}
-	spin_lock_bh(&msk->fallback_lock);
-	if (!msk->allow_infinite_fallback) {
-		spin_unlock_bh(&msk->fallback_lock);
-		return false;
-	}
-
-	msk->allow_subflows = false;
-	set_bit(MPTCP_FALLBACK_DONE, &msk->flags);
-	spin_unlock_bh(&msk->fallback_lock);
-	return true;
-}
-
 static inline bool __mptcp_has_initial_subflow(const struct mptcp_sock *msk)
 {
 	struct sock *ssk = READ_ONCE(msk->first);
@@ -1250,14 +1232,16 @@ static inline bool __mptcp_has_initial_subflow(const struct mptcp_sock *msk)
 			TCPF_SYN_RECV | TCPF_LISTEN));
 }
 
-static inline bool mptcp_try_fallback(struct sock *ssk)
+bool __mptcp_try_fallback(struct mptcp_sock *msk, int fb_mib);
+
+static inline bool mptcp_try_fallback(struct sock *ssk, int fb_mib)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct sock *sk = subflow->conn;
 	struct mptcp_sock *msk;
 
 	msk = mptcp_sk(sk);
-	if (!__mptcp_try_fallback(msk))
+	if (!__mptcp_try_fallback(msk, fb_mib))
 		return false;
 	if (READ_ONCE(msk->snd_data_fin_enable) && !(ssk->sk_shutdown & SEND_SHUTDOWN)) {
 		gfp_t saved_allocation = ssk->sk_allocation;
@@ -1275,12 +1259,13 @@ static inline bool mptcp_try_fallback(struct sock *ssk)
 
 #define pr_fallback(a) pr_debug("%s:fallback to TCP (msk=%p)\n", __func__, a)
 
-static inline void mptcp_subflow_early_fallback(struct mptcp_sock *msk,
-						struct mptcp_subflow_context *subflow)
+static inline void mptcp_early_fallback(struct mptcp_sock *msk,
+					struct mptcp_subflow_context *subflow,
+					int fb_mib)
 {
 	pr_fallback(msk);
 	subflow->request_mptcp = 0;
-	WARN_ON_ONCE(!__mptcp_try_fallback(msk));
+	WARN_ON_ONCE(!__mptcp_try_fallback(msk, fb_mib));
 }
 
 static inline bool mptcp_check_infinite_map(struct sk_buff *skb)
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 1802bc5435a1aaabc81e28152b0bac5656e3b828..600e59bba363f9b3350d9d65ca729cba4badb304 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -544,11 +544,13 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	mptcp_get_options(skb, &mp_opt);
 	if (subflow->request_mptcp) {
 		if (!(mp_opt.suboptions & OPTION_MPTCP_MPC_SYNACK)) {
-			if (!mptcp_try_fallback(sk))
+			if (!mptcp_try_fallback(sk,
+						MPTCP_MIB_MPCAPABLEACTIVEFALLBACK)) {
+				MPTCP_INC_STATS(sock_net(sk),
+						MPTCP_MIB_FALLBACKFAILED);
 				goto do_reset;
+			}
 
-			MPTCP_INC_STATS(sock_net(sk),
-					MPTCP_MIB_MPCAPABLEACTIVEFALLBACK);
 			pr_fallback(msk);
 			goto fallback;
 		}
@@ -1406,7 +1408,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			return true;
 		}
 
-		if (!mptcp_try_fallback(ssk)) {
+		if (!mptcp_try_fallback(ssk, MPTCP_MIB_DSSFALLBACK)) {
 			/* fatal protocol error, close the socket.
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */
@@ -1859,7 +1861,7 @@ static void subflow_state_change(struct sock *sk)
 
 	msk = mptcp_sk(parent);
 	if (subflow_simultaneous_connect(sk)) {
-		WARN_ON_ONCE(!mptcp_try_fallback(sk));
+		WARN_ON_ONCE(!mptcp_try_fallback(sk, MPTCP_MIB_SIMULTCONNFALLBACK));
 		pr_fallback(msk);
 		subflow->conn_finished = 1;
 		mptcp_propagate_state(parent, sk, subflow, NULL);

-- 
2.50.0

[PATCH net-next 2/2] mptcp: remove pr_fallback()

[Matthieu Baerts (NGI0)]

From: Paolo Abeni <pabeni@redhat.com>

We can now track fully the fallback status of a given connection via the
relevant mibs, the mentioned helper is redundant. Remove it completely.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/options.c  | 1 -
 net/mptcp/protocol.c | 1 -
 net/mptcp/protocol.h | 3 ---
 net/mptcp/subflow.c  | 4 ----
 4 files changed, 9 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 6cf02344249a65d00b5832ebb69dd39d6e5fe98e..70c0ab0ecf905d282e5dc6c1b21ffc6476c8d71b 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -982,7 +982,6 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_FALLBACKFAILED);
 			goto reset;
 		}
-		pr_fallback(msk);
 		return false;
 	}
 
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 88bf092f230abbecddc965b4242f711a5ba9d6b6..9a287b75c1b31bac9c35581db996e39e594872e0 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1167,7 +1167,6 @@ static void mptcp_update_infinite_map(struct mptcp_sock *msk,
 	}
 
 	mptcp_subflow_ctx(ssk)->send_infinite_map = 0;
-	pr_fallback(msk);
 }
 
 #define MPTCP_MAX_GSO_SIZE (GSO_LEGACY_MAX_SIZE - (MAX_TCP_HEADER + 1))
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 912f048994a196304a80cde90747d4a512fb3d3d..b15d7fab5c4b66c6fb7a7cbdc91d49547ada6a94 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -1257,13 +1257,10 @@ static inline bool mptcp_try_fallback(struct sock *ssk, int fb_mib)
 	return true;
 }
 
-#define pr_fallback(a) pr_debug("%s:fallback to TCP (msk=%p)\n", __func__, a)
-
 static inline void mptcp_early_fallback(struct mptcp_sock *msk,
 					struct mptcp_subflow_context *subflow,
 					int fb_mib)
 {
-	pr_fallback(msk);
 	subflow->request_mptcp = 0;
 	WARN_ON_ONCE(!__mptcp_try_fallback(msk, fb_mib));
 }
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 600e59bba363f9b3350d9d65ca729cba4badb304..3f1b62a9fe889ab1ac07f2a210b9de050436b37e 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -551,7 +551,6 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 				goto do_reset;
 			}
 
-			pr_fallback(msk);
 			goto fallback;
 		}
 
@@ -1855,14 +1854,11 @@ static void subflow_state_change(struct sock *sk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
 	struct sock *parent = subflow->conn;
-	struct mptcp_sock *msk;
 
 	__subflow_state_change(sk);
 
-	msk = mptcp_sk(parent);
 	if (subflow_simultaneous_connect(sk)) {
 		WARN_ON_ONCE(!mptcp_try_fallback(sk, MPTCP_MIB_SIMULTCONNFALLBACK));
-		pr_fallback(msk);
 		subflow->conn_finished = 1;
 		mptcp_propagate_state(parent, sk, subflow, NULL);
 	}

-- 
2.50.0

Re: [PATCH net-next 0/2] mptcp: track more fallback cases

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 23 Jul 2025 16:32:22 +0200 you wrote:
> This series has two patches linked to fallback to TCP:
> 
> - Patch 1: additional MIB counters for remaining error code paths around
>   fallback
> 
> - Patch 2: remove dedicated pr_debug() linked to fallback now that
>   everything should be covered by dedicated MIB counters.
> 
> [...]

Here is the summary with links:
  - [net-next,1/2] mptcp: track fallbacks accurately via mibs
    https://git.kernel.org/netdev/net-next/c/c65c2e3bae69
  - [net-next,2/2] mptcp: remove pr_fallback()
    https://git.kernel.org/netdev/net-next/c/829fec0244b4

You are awesome, thank you!
-- 
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