[PATCH v3 net-next 0/7] mptcp: implement TCP_NOTSENT_LOWAT support

[Paolo Abeni <pabeni@redhat.com>]

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.

v2 -> v3:
  - typo in patch 1/7
  - dropped unused code in patch 3/7
  - dropped duplicate code in patch 6/7

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

*** BLURB HERE ***

Paolo Abeni (7):
  mptcp: push at 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 | 57 ++++++++++++++++++++++++-----------
 net/mptcp/protocol.h | 51 ++++++++++++++++++++++---------
 net/mptcp/sockopt.c  | 71 ++++++++++++++++++++------------------------
 3 files changed, 108 insertions(+), 71 deletions(-)

-- 
2.43.0