mptcp: improve multiple xmit streams support

This series improves MPTCP handling of multiple concurrent xmit streams.

The to-be-transmitted data is enqueued to a subflow only when the send window is open, keeping the subflows xmit queue shorter and allowing for faster switch-over.

The above requires a more accurate msk socket state tracking and some additional infrastructure to allow pushing the data pending in the msk xmit queue as soon as the MPTCP's send window opens (patches 6-10).

As a side effect, the MPTCP socket could enqueue data to subflows after close() time - to completely spooling the data sitting in the msk xmit queue. Dealing with the requires some infrastructure and core TCP changes (patches 1-5)

Finally, patches 11-12 introduce a more accurate tracking of the other end's receive window.

Overall this refactor the MPTCP xmit path, without introducing new features - the new code is covered by the existing self-tests.

v2 -> v3:
- rebased,
- fixed checkpatch issue in patch 1/13
- fixed some state tracking issues in patch 8/13

v1 -> v2:
- this is just a report, to cope with patchwork issues, no changes at all

Florian Westphal (2):
mptcp: rework poll+nospace handling
mptcp: keep track of advertised windows right edge

Paolo Abeni (11):
tcp: factor out tcp_build_frag()
mptcp: use tcp_build_frag()
tcp: factor out __tcp_close() helper
mptcp: introduce mptcp_schedule_work
mptcp: reduce the arguments of mptcp_sendmsg_frag
mptcp: add accounting for pending data
mptcp: introduce MPTCP snd_nxt
mptcp: refactor shutdown and close
mptcp: move page frag allocation in mptcp_sendmsg()
mptcp: try to push pending data on snd una updates
mptcp: send explicit ack on delayed ack_seq incr

 include/net/tcp.h      |   4 +
 net/ipv4/tcp.c         | 128 +++---
 net/mptcp/options.c    |  30 +-
 net/mptcp/pm.c         |   3 +-
 net/mptcp/pm_netlink.c |   6 +-
 net/mptcp/protocol.c   | 998 ++++++++++++++++++++++++-----------------
 net/mptcp/protocol.h   |  72 ++-
 net/mptcp/subflow.c    |  33 +-
 8 files changed, 776 insertions(+), 498 deletions(-)

2.26.2