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.

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   | 969 ++++++++++++++++++++++++-----------------
 net/mptcp/protocol.h   |  72 ++-
 net/mptcp/subflow.c    |  33 +-
 8 files changed, 758 insertions(+), 487 deletions(-)

-- 
2.26.2