Re: [PATCH mptcp-net] mptcp: fix ack generation for fallback msk

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

On 07/11/2025 01:10, Paolo Abeni wrote:
> mptcp_cleanup_rbuf() need to know the last most recent, mptcp-level
> rcv_wnd sent, and such information is tracked into the msk->old_wspace
> field, updated at ack transmission time by mptcp_write_options().
> 
> Fallback socket do not add any mptcp options, such helper is never
> invoked, and msk->old_wspace value remain stale. That in turn makes
> ack generation at recvmsg() time quite randomic.
> 
> Address the issue ensuring mptcp_write_options() is invoked even
> for fallback sockets, and just update the needed info in such a
> case.
> 
> The issue went unnoticed for a long time, as mptcp overshot the> fallback socket receive buffer autotune significantly.

Should we say "used to overshot"? :)

> Fixes: e3859603ba13 ("mptcp: better msk receive window updates")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/594

Thank you for the fix!

(...)

> @@ -1643,6 +1660,9 @@ void mptcp_write_options(struct tcphdr *th, __be32 *ptr, struct tcp_sock *tp,
>  				      opts->reset_transient,
>  				      opts->reset_reason);
>  		return;
> +	} else if (unlikely(!opts->suboptions)) {

Small detail: do you mind if I add /* Fallback to TCP */ just above when
applying the patch?

> +		mptcp_track_rwin(tp);

Should we eventually also call mptcp_track_rwin(tp) in case of MP_FAIL
without MP_RST? Or is this not needed?

Other than that, it looks good to me, and I guess we can already apply
the patch:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

> +		return;
>  	}
>  
>  	if (OPTION_MPTCP_PRIO & opts->suboptions) {

Cheers,
Matt
-- 
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