This an attempt to fix the data transfer stall reported by Geliang and
Gang more carefully enforcing memory constraints at the MPTCP level.

The first path is from Shardul, included here as the condition addressed
by such a patch is frequently hit in the relevant test-cased.

Patch 2 and 3 makes the admission check much more strict for incoming
packets exceeding the memory limits, with some exception for fallback
sockets.
Patch 4, 5, 6 and 7 are cleanups/refactors finalized to safely re-using
TCP helpers on MPTCP skbs.
Patch 8 makes TCP pruning related helpers available to MPTCP and patch 9
makes use of them. Patch 10 addresses an edge scenario that could still
lead to transfer stall under memory pressure.
Finally patch 11 and 12 improve the MPTCP-level retransmission schema to
make recovery from memory pressure significanly faster.

Tested successfully vs the test cases proposed by Geliang and Gang and
vs the selftests.
---
Jast a few minor updates over v4 (that was misnamed without revision) to
deal with sashiko's feedback (some was irrelevant/wrong, mentioned as
reported on the previous revision). Changes described in the individual
patches.
Patch 1 should go via net, included here for my own sake :-P

*** BLURB HERE ***

Paolo Abeni (12):
  mptcp: do not drop partial packets
  mptcp: explicitly drop over memory limits
  mptcp: enforce hard limit on backlog flushing
  mptcp: drop the mptcp_ooo_try_coalesce() helper
  mptcp: drop the cant_coalesce CB field
  mptcp: remove CB offset field
  mptcp: sync mptcp skb cb layout with tcp one
  tcp: expose the tcp_collapse_ofo_queue() helper to mptcp usage, too
  mptcp: implemented OoO queue pruning
  mptcp: track prune recovery status
  mptcp: move the retrans loop to a separate helper
  mptcp: let the retrans scheduler do its job.

 include/net/tcp.h    |   8 +
 net/ipv4/tcp_input.c |  54 +++--
 net/mptcp/fastopen.c |  17 +-
 net/mptcp/mib.c      |   3 +
 net/mptcp/mib.h      |   3 +
 net/mptcp/options.c  |  74 ++++++-
 net/mptcp/protocol.c | 487 +++++++++++++++++++++++++++++--------------
 net/mptcp/protocol.h |  24 ++-
 net/mptcp/subflow.c  |  11 +
 9 files changed, 492 insertions(+), 189 deletions(-)

-- 
2.54.0

Hi Paolo,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Unstable: 2 failed test(s): packetdrill_fastopen packetdrill_sockopts ⚠️ 
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/25640183335

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/49dce7f2f3dd
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1092385


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)