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

[Simon Baatz]

Hi Matt,

On Wed, Mar 11, 2026 at 07:27:34PM +0100, Matthieu Baerts wrote:
> Hi Simon,
> 
> On 09/03/2026 09:02, Simon Baatz via B4 Relay wrote:
> > From: Simon Baatz <gmbnomis@xxxxxxxxx>
> > 
> > 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!
> 
> > Signed-off-by: Simon Baatz <gmbnomis@xxxxxxxxx>
> > ---
> >  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
> 
> (...)
> 
> > @@ -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).  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.

So, I changed it for legibility.  Now, I realize it made the
diff harder to read.  I might have optimized the wrong metric here ;-)

> 
> > +		tcp_update_max_rcv_wnd_seq(tp);
> 
> 
> This patch adding this new helper each time rcv_wnd is modified looks
> good to me:
> 
> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
> 
> 
> Note: just in case a new version is needed, checkpatch reported an error
> in patch 4/6 because of a trailing whitespace (+ No space is necessary
> after a cast in patch 1/6), see:
> 
>   https://github.com/multipath-tcp/mptcp_net-next/actions/runs/22844479818

Thanks. I will change that if there is a v4.


-- 
Simon Baatz <gmbnomis@xxxxxxxxx>