[PATCH net-next v2 0/4] mptcp: add TCP_MAXSEG sockopt support

[PATCH net-next v2 0/4] mptcp: add TCP_MAXSEG sockopt support

[Matthieu Baerts (NGI0)]

The TCP_MAXSEG socket option was not supported by MPTCP, mainly because
it has never been requested before. But there are still valid use-cases,
e.g. with HAProxy.

- Patch 1 is a small cleanup patch in the MPTCP sockopt file.

- Patch 2 expose some code from TCP, to avoid duplicating it in MPTCP.

- Patch 3 adds TCP_MAXSEG sockopt support in MPTCP.

- Patch 4 is not related to the others, it fixes a typo in a comment.

Note that the new TCP_MAXSEG sockopt support has been validated by a new
packetdrill script on the MPTCP CI:

  https://github.com/multipath-tcp/packetdrill/pull/161

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Changes in v2:
- Patch 2: Set the return value of tcp_sock_set_maxseg() to err.
- Patch 3: make mptcp_setsockopt_all_sf() returns only once.
- Link to v1: https://lore.kernel.org/r/20250716-net-next-mptcp-tcp_maxseg-v1-0-548d3a5666f6@kernel.org

---
Geliang Tang (3):
      mptcp: sockopt: drop redundant tcp_getsockopt
      tcp: add tcp_sock_set_maxseg
      mptcp: add TCP_MAXSEG sockopt support

moyuanhao (1):
      mptcp: fix typo in a comment

 include/linux/tcp.h  |  1 +
 net/ipv4/tcp.c       | 23 ++++++++++++++---------
 net/mptcp/protocol.c |  2 +-
 net/mptcp/protocol.h |  1 +
 net/mptcp/sockopt.c  | 33 +++++++++++++++++++++++++++++----
 5 files changed, 46 insertions(+), 14 deletions(-)
---
base-commit: d61f6cb6f6ef3c70d2ccc0d9c85c508cb8017da9
change-id: 20250716-net-next-mptcp-tcp_maxseg-e7702891080d

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

[PATCH net-next v2 1/4] mptcp: sockopt: drop redundant tcp_getsockopt

[Matthieu Baerts (NGI0)]

From: Geliang Tang <tanggeliang@kylinos.cn>

tcp_getsockopt() is called twice in mptcp_getsockopt_first_sf_only() in
different conditions, which makes the code a bit redundant.

The first call to tcp_getsockopt() when the first subflow exists can be
replaced by going to a new label "get" before the second call.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/sockopt.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 3caa0a9d3b3885ce6399570f2d98a2e8f103638d..afa54fba51e215bc2efb21f16ed7d0a0fb120972 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -914,10 +914,8 @@ static int mptcp_getsockopt_first_sf_only(struct mptcp_sock *msk, int level, int
 
 	lock_sock(sk);
 	ssk = msk->first;
-	if (ssk) {
-		ret = tcp_getsockopt(ssk, level, optname, optval, optlen);
-		goto out;
-	}
+	if (ssk)
+		goto get;
 
 	ssk = __mptcp_nmpc_sk(msk);
 	if (IS_ERR(ssk)) {
@@ -925,6 +923,7 @@ static int mptcp_getsockopt_first_sf_only(struct mptcp_sock *msk, int level, int
 		goto out;
 	}
 
+get:
 	ret = tcp_getsockopt(ssk, level, optname, optval, optlen);
 
 out:

-- 
2.50.0

[PATCH net-next v2 2/4] tcp: add tcp_sock_set_maxseg

[Matthieu Baerts (NGI0)]

From: Geliang Tang <tanggeliang@kylinos.cn>

Add a helper tcp_sock_set_maxseg() to directly set the TCP_MAXSEG
sockopt from kernel space.

This new helper will be used in the following patch from MPTCP.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
- v2: Set the return value of tcp_sock_set_maxseg() to err.
---
 include/linux/tcp.h |  1 +
 net/ipv4/tcp.c      | 23 ++++++++++++++---------
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 1a5737b3753d06165bc71e257a261bcd7a0085ce..57e478bfaef20369f5dba1cff540e52c9302ebf4 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -621,6 +621,7 @@ void tcp_sock_set_nodelay(struct sock *sk);
 void tcp_sock_set_quickack(struct sock *sk, int val);
 int tcp_sock_set_syncnt(struct sock *sk, int val);
 int tcp_sock_set_user_timeout(struct sock *sk, int val);
+int tcp_sock_set_maxseg(struct sock *sk, int val);
 
 static inline bool dst_tcp_usec_ts(const struct dst_entry *dst)
 {
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 31149a0ac849192b46c67dd569efeeeb0a041a0b..71a956fbfc5533224ee00e792de2cfdccd4d40aa 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3751,6 +3751,19 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	return 0;
 }
 
+int tcp_sock_set_maxseg(struct sock *sk, int val)
+{
+	/* Values greater than interface MTU won't take effect. However
+	 * at the point when this call is done we typically don't yet
+	 * know which interface is going to be used
+	 */
+	if (val && (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW))
+		return -EINVAL;
+
+	tcp_sk(sk)->rx_opt.user_mss = val;
+	return 0;
+}
+
 /*
  *	Socket option code for TCP.
  */
@@ -3883,15 +3896,7 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 
 	switch (optname) {
 	case TCP_MAXSEG:
-		/* Values greater than interface MTU won't take effect. However
-		 * at the point when this call is done we typically don't yet
-		 * know which interface is going to be used
-		 */
-		if (val && (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW)) {
-			err = -EINVAL;
-			break;
-		}
-		tp->rx_opt.user_mss = val;
+		err = tcp_sock_set_maxseg(sk, val);
 		break;
 
 	case TCP_NODELAY:

-- 
2.50.0

[PATCH net-next v2 3/4] mptcp: add TCP_MAXSEG sockopt support

[Matthieu Baerts (NGI0)]

From: Geliang Tang <tanggeliang@kylinos.cn>

The TCP_MAXSEG socket option is currently not supported by MPTCP, mainly
because it has never been requested before. But there are still valid
use-cases, e.g. with HAProxy.

This patch adds its support in MPTCP by propagating the value to all
subflows. The get part looks at the value on the first subflow, to be as
closed as possible to TCP. Only one value can be returned for the cached
MSS, so this can come only from one subflow.

Similar to mptcp_setsockopt_first_sf_only(), a generic helper
mptcp_setsockopt_all_subflows() is added to set sockopt for each
subflows of the mptcp socket.

Add a new member for struct mptcp_sock to store the TCP_MAXSEG value,
and return this value in getsockopt.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/515
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
- v2: make mptcp_setsockopt_all_sf() returns only once.
---
 net/mptcp/protocol.h |  1 +
 net/mptcp/sockopt.c  | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 6ec245fd2778ef30e0dc84d309d27ecd1f62e0d1..1a32edf6f34364eedd5d077eae7a82a7db8c3a9e 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -326,6 +326,7 @@ struct mptcp_sock {
 	int		keepalive_cnt;
 	int		keepalive_idle;
 	int		keepalive_intvl;
+	int		maxseg;
 	struct work_struct work;
 	struct sk_buff  *ooo_last_skb;
 	struct rb_root  out_of_order_queue;
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index afa54fba51e215bc2efb21f16ed7d0a0fb120972..2c267aff95bec90d341de9f9e136b9d2df9b8e13 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -798,6 +798,23 @@ static int mptcp_setsockopt_first_sf_only(struct mptcp_sock *msk, int level, int
 	return ret;
 }
 
+static int mptcp_setsockopt_all_sf(struct mptcp_sock *msk, int level,
+				   int optname, sockptr_t optval,
+				   unsigned int optlen)
+{
+	struct mptcp_subflow_context *subflow;
+	int ret = 0;
+
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		ret = tcp_setsockopt(ssk, level, optname, optval, optlen);
+		if (ret)
+			break;
+	}
+	return ret;
+}
+
 static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 				    sockptr_t optval, unsigned int optlen)
 {
@@ -859,6 +876,11 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 						 &msk->keepalive_cnt,
 						 val);
 		break;
+	case TCP_MAXSEG:
+		msk->maxseg = val;
+		ret = mptcp_setsockopt_all_sf(msk, SOL_TCP, optname, optval,
+					      optlen);
+		break;
 	default:
 		ret = -ENOPROTOOPT;
 	}
@@ -1406,6 +1428,9 @@ static int mptcp_getsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		return mptcp_put_int_option(msk, optval, optlen, msk->notsent_lowat);
 	case TCP_IS_MPTCP:
 		return mptcp_put_int_option(msk, optval, optlen, 1);
+	case TCP_MAXSEG:
+		return mptcp_getsockopt_first_sf_only(msk, SOL_TCP, optname,
+						      optval, optlen);
 	}
 	return -EOPNOTSUPP;
 }
@@ -1552,6 +1577,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
 	tcp_sock_set_keepidle_locked(ssk, msk->keepalive_idle);
 	tcp_sock_set_keepintvl(ssk, msk->keepalive_intvl);
 	tcp_sock_set_keepcnt(ssk, msk->keepalive_cnt);
+	tcp_sock_set_maxseg(ssk, msk->maxseg);
 
 	inet_assign_bit(TRANSPARENT, ssk, inet_test_bit(TRANSPARENT, sk));
 	inet_assign_bit(FREEBIND, ssk, inet_test_bit(FREEBIND, sk));

-- 
2.50.0

[PATCH net-next v2 4/4] mptcp: fix typo in a comment

[Matthieu Baerts (NGI0)]

From: moyuanhao <moyuanhao3676@163.com>

This patch fixes the follow spelling mistake in a comment:

  greter -> greater

Signed-off-by: moyuanhao <moyuanhao3676@163.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
 - The same patch has already been sent to the netdev ML, but when
   net-next was closed:
   https://lore.kernel.org/20250530181004.261417-1-moyuanhao3676@163.com
---
 net/mptcp/protocol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 2ad1c41e963ec30cccd8387201f622c4f4d4c471..6c448a0be9495b22ced4a2b51da2f80831040aba 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1387,7 +1387,7 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 	 * - estimate the faster flow linger time
 	 * - use the above to estimate the amount of byte transferred
 	 *   by the faster flow
-	 * - check that the amount of queued data is greter than the above,
+	 * - check that the amount of queued data is greater than the above,
 	 *   otherwise do not use the picked, slower, subflow
 	 * We select the subflow with the shorter estimated time to flush
 	 * the queued mem, which basically ensure the above. We just need

-- 
2.50.0

Re: [PATCH net-next v2 0/4] mptcp: add TCP_MAXSEG sockopt support

[patchwork-bot+netdevbpf]

Hello:

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

On Sat, 19 Jul 2025 00:06:55 +0200 you wrote:
> The TCP_MAXSEG socket option was not supported by MPTCP, mainly because
> it has never been requested before. But there are still valid use-cases,
> e.g. with HAProxy.
> 
> - Patch 1 is a small cleanup patch in the MPTCP sockopt file.
> 
> - Patch 2 expose some code from TCP, to avoid duplicating it in MPTCP.
> 
> [...]

Here is the summary with links:
  - [net-next,v2,1/4] mptcp: sockopt: drop redundant tcp_getsockopt
    https://git.kernel.org/netdev/net-next/c/edd669057c56
  - [net-next,v2,2/4] tcp: add tcp_sock_set_maxseg
    https://git.kernel.org/netdev/net-next/c/51a62199a8aa
  - [net-next,v2,3/4] mptcp: add TCP_MAXSEG sockopt support
    https://git.kernel.org/netdev/net-next/c/51c5fd09e1b4
  - [net-next,v2,4/4] mptcp: fix typo in a comment
    https://git.kernel.org/netdev/net-next/c/154e56a77d81

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html