mptcp: give rcvlowat some love
The MPTCP protocol allow setting sk_rcvlowat, but the value there is currently ignored. Additionally, the default subflows sk_rcvlowat basically disables per subflow delayed ack: the MPTCP protocol move the incoming data from the subflows into the msk socket as soon as the TCP stacks invokes the subflow data_ready callback. Later, when __tcp_ack_snd_check() takes action, the subflow-level copied_seq matches rcv_nxt, and that mandate for an immediate ack. Let the mptcp receive path be aware of such threshold, explicitly tracking the amount of data available to be ready and checking vs sk_rcvlowat in mptcp_poll() and before waking-up readers. Additionally implement the set_rcvlowat() callback, to properly handle the rcvbuf auto-tuning on sk_rcvlowat changes. Finally to properly handle delayed ack, force the subflow level threshold to 0 and instead explicitly ask for an immediate ack when the msk level th is not reached. Reviewed-by:Mat Martineau <martineau@kernel.org> Signed-off-by: