Re: [PATCH v3 mptcp-next 2/7] mptcp: avoid unneeded subflow-level drops.

[Matthieu Baerts <matttbe@kernel.org>]

On 11/11/2025 19:43, Matthieu Baerts wrote:
> Hi Paolo,
> 
> On 07/11/2025 09:32, Paolo Abeni wrote:
>> The rcv window is shared among all the subflows. Currently, MPTCP sync
>> the TCP-level rcv window with the MPTCP one at tcp_transmit_skb() time.
>>
>> The above means that incoming data may sporadically observe outdated
>> TCP-level rcv window and being wrongly dropped by TCP.
>>
>> Address the issue checking for the edge condition before queuing the data
>> at TCP level, and eventually syncing the rcv window as needed.
>>
>> Note that the issue is actually present from the very first MPTCP
>> implementation, but backports older than the blamed commit below will range
>> from impossible to useless.
> 
> Thank you for this patch!
> 
> It looks good to me:
> 
> Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> 
>> Before:
>> nstat >/dev/null ;sleep 1; nstat -z TcpExtBeyondWindow
>> TcpExtBeyondWindow              14                 0.0
>>
>> After:
>> nstat >/dev/null ;sleep 1; nstat -z TcpExtBeyondWindow
>> TcpExtBeyondWindow              0                  0.0
> 
> Should we eventually track this MIB counter in mptcp_connect.sh?

(This can be done in another patch later, we can also create a task to
do it later if that's easier of course.)

> 
>> Fixes: fa3fe2b15031 ("mptcp: track window announced to peer")
> 
> While there are still discussions on patch 6/7, to ease things, I
> suggest already applying only this patch in our tree (fixes for -net),
> the only material for 'net' from what I understood.

Done:

New patches for t/upstream-net and t/upstream:
- 9250eb1bd82b: mptcp: avoid unneeded subflow-level drops
- Results: 7d372ea35141..25f5033b9887 (export-net)
- Results: 3b1f54eb2ace..3dad323fd8af (export)

Tests are now in progress:

- export-net:
https://github.com/multipath-tcp/mptcp_net-next/commit/ef271ef4961b23166a56d80c5136bb85b29826d8/checks
- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/473f3b6a6838866eed83d85449df20f654ac4c1a/checks

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.