Re: [PATCH net-next v3 2/6] mptcp: keep rcv_mwnd_seq in sync with subflow rcv_wnd

Hi Simon,

On 11/03/2026 23:08, Simon Baatz wrote:

On Wed, Mar 11, 2026 at 07:27:34PM +0100, Matthieu Baerts wrote:

quoted

On 09/03/2026 09:02, Simon Baatz via B4 Relay wrote:

quoted

From: Simon Baatz <redacted>

MPTCP shares a receive window across subflows and applies it at the
subflow level by adjusting each subflow's rcv_wnd when needed.  With
the new TCP tracking of the maximum advertised window sequence,
rcv_mwnd_seq must stay consistent with these subflow-level rcv_wnd
adjustments.

Thank you for these modifications!

quoted

Signed-off-by: Simon Baatz <redacted>
---
 net/mptcp/options.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 43df4293f58bfbd8a8df6bf24b9f15e0f9e238f6..8a1c5698983cff3082d68290626dd8f1e044527f 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c

(...)

quoted

@@ -1338,8 +1339,9 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 		 */
 		rcv_wnd_new = rcv_wnd_old;
 		win = rcv_wnd_old - ack_seq;
-		tp->rcv_wnd = min_t(u64, win, U32_MAX);
-		new_win = tp->rcv_wnd;
+		new_win = min_t(u64, win, U32_MAX);
+		tp->rcv_wnd = new_win;

Out of curiosity, why did you change the two lines above?
(even if it makes sense, the diff is a bit confusing, and the commit
message doesn't mention this :) )

I wanted to keep tcp_update_max_rcv_wnd_seq() calls close to the
respective update sites (same pattern everywhere).

Thanks, I now understand the reason.

In the original form

tp->rcv_wnd = min_t(u64, win, U32_MAX);
tcp_update_max_rcv_wnd_seq(tp);
new_win = tp->rcv_wnd;

the ordering suggests that tcp_update_max_rcv_wnd_seq() might modify
tp->rcv_wnd.

Note that if tp->rcv_mwnd_seq always needs to be modified when
tp->rcv_wnd and/or tp->rcv_wup are modified, maybe a single helper could
be called to modify all of them, so it might be less likely to forget
about modifying tp->rcv_mwnd_seq as well in the future.

But probably it might be unlikely to have new places where tp->rcv_wnd
and/or tp->rcv_wup need to be modified like here with MPTCP. So probably
fine like that.

Cheers,
Matt
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