Re: [PATCH export v3 4/4] mptcp: fallback to TCP on MP_FAIL with a single subflow

[Chenguang Zhao <chenguang.zhao@linux.dev>]

在 2026/8/12 18:47, Paolo Abeni 写道:
> On 8/12/26 7:46 AM, Chenguang Zhao wrote:
>> @@ -901,6 +898,12 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
>>  		subflow->send_mp_fail = 1;
>>  		subflow->send_infinite_map = 1;
>>  		tcp_send_ack(sk);
> Slightly unrelated... AFAICS the current code sends the mp_fail reply
> immediatally, and will send the infinite mapping with the later data
> send, if any.
>
> I *think* it will be better to send MP_FAIL and infinite mapping
> together. It should remove a little bit of complexity on the xmit path
> and will be IMHO closer to the RFC specs.
>
> /P
>
To keep both MPTCP options on the same packet, that packet would have
to be sent before FALLBACK_DONE: once fallback is done,
mptcp_established_options() drops MPTCP options unless the skb
already carries an infinite mapping.

tcp_send_ack() builds a plain ACK with no mpext->infinite_map, so it
can include MP_FAIL but not the infinite mapping. The infinite
mapping is filled in later on a data skb in mptcp_sendmsg_frag().
With the current xmit path those two options therefore cannot go out
on the same packet. Sending them together would need a dedicated
transmit helper that builds a skb carrying both, rather than reusing
tcp_send_ack().