Re: [PATCH mptcp-next v2 3/3] selftests: mptcp: sockopt: add TCP_MAXSEG sockopt tests

[Matthieu Baerts <matttbe@kernel.org>]

Hi Geliang

On 23/04/2025 11:40, Geliang Tang wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> This patch adds the TCP_MAXSEG sockopt tests in mptcp_sockopt.c. Since
> in getsockopt TCP_MAXSEG, the "user_mss" value can be obtained only in
> the LISTEN state (see do_tcp_getsockopt in net/ipv4/tcp.c), the test
> items are added to server() instead of client().

I understand you wanted a way to test it, but the set/get is not enough,
and it looks "useless". I'm OK to apply the two other patches with the
small suggested modifications, but I will only send them when there will
be a packetdrill test.

Do not hesitate to imitate other packetdrill test, e.g.
mptcp/sockopts/sockopt_keepalive.pkt. In this case, you could set MAXSEG
(e.g. 1000), then send data (e.g. write(5000)) and check 4 packets are
sent, and the MSS is set to 1000. It might be good to change the value
again later, and see if the behaviour is the expected one
(setsockopt(MAXSEG, 1250) + write(2500)). If possible, after that, in
the same test, also create another subflow, and only check the MSS to
see if it has the expected value. To test that, feel free to look at:

https://github.com/multipath-tcp/mptcp-upstream-virtme-docker?tab=readme-ov-file#packetdrill

Note that the behaviour should be as closed as possible to TCP. Don't
hesitate to run the same kind of test with "plain" TCP.

Cheers,
Matt
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