Patch 6/7 does the magic, all the others are minor cleanup and fix of
buglet exposed by such feature. I'll push a paired pktdrill test.

Note that this relies on the existing accounting for snd_nxt. As I
stated such accounting is not 110% accurate as it tracks the most recent
sequence number queued to any subflow, and not the actual sequence
number sent on the wire.

I experimented a lot trying to implement the latter and in the end it
proved to be both "too complex" and "not necessary".

The complexity raises from the need for additional lock and a lot of
refactory to introduce such protection without adding significant
overhead. Additionally, snd_nxt is currenly used and exposed with the
current semantic both eBPF and the internal packet scheduling.
Introducing a different tracking will still require us to keep the old
one.

More interesting, a more accurate tracking could be not strictly
necessary: as the MPTCP protocol enqueues data to the subflows only up
the available send window, any enqueue data is sent on the wire
instantly, without any blocking operation short of a drop in the tx path
at the nft or TC layer.

The individual patches changelog carry the gory details.

Still sending a single series to outline the functional requirements,
even if the first 3 patches could land on mptcp-net directly.

v1 -> v2:
  - clarifiy commit message in patch 1/7
  - fix possible wake-up bug in patch 6/7

Paolo Abeni (7):
  mptcp: push ad DSS boundaries
  mptcp: fix snd_wnd initialization for passive socket
  mptcp: fix potential wake-up event loss
  mptcp: cleanup writer wake-up
  mptcp: avoid some duplicate code in socket option handling
  mptcp: implement TCP_NOTSENT_LOWAT support.
  mptcp: cleanup SOL_TCP handling

 net/mptcp/protocol.c | 66 +++++++++++++++++++++++++++++-----------
 net/mptcp/protocol.h | 57 ++++++++++++++++++++++++++++-------
 net/mptcp/sockopt.c  | 71 ++++++++++++++++++++------------------------
 3 files changed, 126 insertions(+), 68 deletions(-)

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2.43.0