Re: [PATCH mptcp-net v2 1/2] mptcp: pm: avoid sending RM_ADDR over same subflow

[Mat Martineau <martineau@kernel.org>]

On Fri, 20 Feb 2026, Matthieu Baerts (NGI0) wrote:

> RM_ADDR are sent over an active subflow, the first one in the subflows
> list. There is then a high chance the initial subflow is picked. With
> the in-kernel PM, when an endpoint is removed, a RM_ADDR is sent, then
> linked subflows are closed. This is done for each active MPTCP
> connection.
>
> MPTCP endpoints are likely removed because the attached network is no
> longer available or usable. In this case, it is better to avoid sending
> this RM_ADDR over the subflow that is going to be removed, but prefer
> sending it over another active and non stale subflow, if any.
>
> This modification avoids situations where the other end is not notified
> when a subflow is no longer usable: typically when the endpoint linked
> to the initial subflow is removed, especially on the server side.
>
> Fixes: 8dd5efb1f91b ("mptcp: send ack for rm_addr")
> Reported-by: Frank Lorenz
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/612
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---
> Note: in my initial version, I only used one alternative for both
> "stale" and "same id" subflows. I guess it is better to send over the
> same subflow than a stale one, hence the priority, but there are then a
> few more lines of code (but still readable, I think). To be discussed.
>
> v2:
> - reduce one indentation level and s/rlist/rm_list/g
> ---
> net/mptcp/pm.c | 55 +++++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 43 insertions(+), 12 deletions(-)
>
> diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
> index 8206b0fd2377..daef91e597ae 100644
> --- a/net/mptcp/pm.c
> +++ b/net/mptcp/pm.c
> @@ -212,9 +212,24 @@ void mptcp_pm_send_ack(struct mptcp_sock *msk,
> 	spin_lock_bh(&msk->pm.lock);
> }
>
> -void mptcp_pm_addr_send_ack(struct mptcp_sock *msk)
> +static bool subflow_in_rm_list(const struct mptcp_subflow_context *subflow,
> +			       const struct mptcp_rm_list *rm_list)
> {
> -	struct mptcp_subflow_context *subflow, *alt = NULL;
> +	u8 i, id = subflow_get_local_id(subflow);
> +
> +	for (i = 0; i < rm_list->nr; i++) {
> +		if (rm_list->ids[i] == id)
> +			return true;
> +	}
> +
> +	return false;
> +}
> +
> +static void
> +mptcp_pm_addr_send_ack_avoid_list(struct mptcp_sock *msk,
> +				  const struct mptcp_rm_list *rm_list)
> +{
> +	struct mptcp_subflow_context *subflow, *stale = NULL, *same_id = NULL;
>
> 	msk_owned_by_me(msk);
> 	lockdep_assert_held(&msk->pm.lock);
> @@ -224,19 +239,35 @@ void mptcp_pm_addr_send_ack(struct mptcp_sock *msk)
> 		return;
>
> 	mptcp_for_each_subflow(msk, subflow) {
> -		if (__mptcp_subflow_active(subflow)) {
> -			if (!subflow->stale) {
> -				mptcp_pm_send_ack(msk, subflow, false, false);
> -				return;
> -			}
> +		if (!__mptcp_subflow_active(subflow))
> +			continue;
>
> -			if (!alt)
> -				alt = subflow;
> +		if (unlikely(subflow->stale)) {
> +			if (!stale)
> +				stale = subflow;
> +		} else if (unlikely(rm_list &&
> +				    subflow_in_rm_list(subflow, rm_list))) {
> +			if (!same_id)
> +				same_id = subflow;
> +		} else {
> +			goto send_ack;

Hi Matthieu -

This is definitely an improvement over the older code, thanks! It does 
still send RM_ADDR exactly once. It could also RM_ADDR using *all* active 
non-stale subflows (any that are delivered after the first would be 
ignored). In terms of interoperability there is the risk of confusing the 
peer's path manager if it doesn't handle RM_ADDR for a non-existant 
subflow.

Maybe that's more of a mptcp-next feature (if it makes sense to do at 
all).

The v2 patch here is closer to the existing behavior so I'm ok with 
approving it:

Reviewed-by: Mat Martineau <martineau@kernel.org>

> 		}
> 	}
>
> -	if (alt)
> -		mptcp_pm_send_ack(msk, alt, false, false);
> +	if (same_id)
> +		subflow = same_id;
> +	else if (stale)
> +		subflow = stale;
> +	else
> +		return;
> +
> +send_ack:
> +	mptcp_pm_send_ack(msk, subflow, false, false);
> +}
> +
> +void mptcp_pm_addr_send_ack(struct mptcp_sock *msk)
> +{
> +	mptcp_pm_addr_send_ack_avoid_list(msk, NULL);
> }
>
> int mptcp_pm_mp_prio_send_ack(struct mptcp_sock *msk,
> @@ -470,7 +501,7 @@ int mptcp_pm_remove_addr(struct mptcp_sock *msk, const struct mptcp_rm_list *rm_
> 	msk->pm.rm_list_tx = *rm_list;
> 	rm_addr |= BIT(MPTCP_RM_ADDR_SIGNAL);
> 	WRITE_ONCE(msk->pm.addr_signal, rm_addr);
> -	mptcp_pm_addr_send_ack(msk);
> +	mptcp_pm_addr_send_ack_avoid_list(msk, rm_list);
> 	return 0;
> }
>
>
> -- 
> 2.51.0
>
>
>