Re: [PATCH mptcp-net] mptcp: update window_clamp on subflows when SO_RCVBUF is set

[gang.yan@linux.dev]

April 16, 2026 at 8:29 PM, "Gang Yan" <gang.yan@linux.dev mailto:gang.yan@linux.dev?to=%22Gang%20Yan%22%20%3Cgang.yan%40linux.dev%3E > wrote:


> 
> From: Gang Yan <yangang@kylinos.cn>
> 
> Add __mptcp_subflow_set_rcvbuf() helper that calls the subflow's
> ops->set_rcvbuf to update window_clamp, in addition to writing
> sk_rcvbuf. Use it in both mptcp_sol_socket_sync_intval() (setsockopt
> path) and sync_socket_options() (new subflow creation path).
> 
Sorry for the confusion in my commit message.

I need to clarify that we cannot call ops->set_rcvbuf() on the MPTCP
subflows as mentioned in the commit message. I will fix this mistake
and update the commit message properly in the next version.

Thank you for your understanding.

Thanks
Gang
> Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/619
> Signed-off-by: Gang Yan <yangang@kylinos.cn>
> ---
> Notes:
> 
> Hi Matt:
> 
> Note that I do not implement set_rcvbuf in mptcp_stream_ops for the 
> following reasons:
> 
>  - The callback receives the MPTCP meta socket as 'sk', but
>  tcp_set_rcvbuf() must be called on each TCP subflow to update
>  window_clamp. A callback implementation would need to iterate all
>  subflows, duplicating the work already done by
>  mptcp_sol_socket_sync_intval().
> 
>  - Subflows are grafted to the MPTCP socket via mptcp_sock_graft(),
>  so ssk->sk_socket->ops points to mptcp_stream_ops, not
>  inet_stream_ops. This means the TCP set_rcvbuf callback is not
>  reachable through the subflow's ops either.
> 
>  - The existing subflow sync functions cover both the setsockopt path
>  (mptcp_sol_socket_sync_intval) and new subflow creation path
>  (sync_socket_options), making the proto_ops callback unnecessary.
> 
> WDYT？
> 
> The packetdrill tests is also implemented:
>  -Link: https://github.com/multipath-tcp/packetdrill/pull/194
> 
> Thanks
> Gang
> 
> ---
>  net/mptcp/sockopt.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
> index de90a2897d2d..abe63392826b 100644
> --- a/net/mptcp/sockopt.c
> +++ b/net/mptcp/sockopt.c
> @@ -67,6 +67,12 @@ static int mptcp_get_int_option(struct mptcp_sock *msk, sockptr_t optval,
>  return 0;
>  }
>  
> +static inline void __mptcp_subflow_set_rcvbuf(struct sock *ssk, int val)
> +{
> + WRITE_ONCE(ssk->sk_rcvbuf, val);
> + tcp_set_rcvbuf(ssk, val);
> +}
> +
>  static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, int val)
>  {
>  struct mptcp_subflow_context *subflow;
> @@ -100,7 +106,7 @@ static void mptcp_sol_socket_sync_intval(struct mptcp_sock *msk, int optname, in
>  case SO_RCVBUF:
>  case SO_RCVBUFFORCE:
>  ssk->sk_userlocks |= SOCK_RCVBUF_LOCK;
> - WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
> + __mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
>  break;
>  case SO_MARK:
>  if (READ_ONCE(ssk->sk_mark) != sk->sk_mark) {
> @@ -1556,7 +1562,7 @@ static void sync_socket_options(struct mptcp_sock *msk, struct sock *ssk)
>  mptcp_subflow_ctx(ssk)->cached_sndbuf = sk->sk_sndbuf;
>  }
>  if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
> - WRITE_ONCE(ssk->sk_rcvbuf, sk->sk_rcvbuf);
> + __mptcp_subflow_set_rcvbuf(ssk, sk->sk_rcvbuf);
>  }
>  
>  if (sock_flag(sk, SOCK_LINGER)) {
> -- 
> 2.43.0
>