From: Geliang Tang <tanggeliang@kylinos.cn>

This series adds MSG_ZEROCOPY support for MPTCP sockets, allowing
userspace to transmit data without intermediate kernel copies, and
provides corresponding selftests to exercise the new path.

The first patch is a small cleanup that introduces a local 'tp'
variable in mptcp_sendmsg_frag() to avoid repeated tcp_sk()
dereferencing, preparing for the subsequent zero-copy changes.

The second patch implements the core MSG_ZEROCOPY support. The design
follows TCP's reference-counting model, with a single ubuf_info per
sendmsg tracked by three reference buckets: one held by sendmsg itself,
one per MPTCP dfrag in the retransmission queue, and one per subflow
skb. Completion is reported only after all bytes are acknowledged at
both MPTCP and subflow levels. The feature is silently downgraded to
regular copy in cases where zero-copy cannot be safely performed
(MSG_FASTOPEN, fallback mode, or memory pressure).

The third and fourth patches add a new 'zerocopy' I/O mode to the
mptcp_connect selftest, and a wrapper script to run it as part of the
regular test suite, ensuring the zero-copy path is exercised in CI.

Geliang Tang (4):
  mptcp: use local variable tp in sendmsg_frag
  mptcp: add msg zerocopy support
  selftests: mptcp: connect: add zerocopy io mode
  selftests: mptcp: connect: cover zerocopy mode

 net/mptcp/protocol.c                          | 104 ++++++++++++++++--
 net/mptcp/protocol.h                          |   1 +
 tools/testing/selftests/net/mptcp/Makefile    |   1 +
 .../selftests/net/mptcp/mptcp_connect.c       |  63 ++++++++++-
 .../net/mptcp/mptcp_connect_zerocopy.sh       |   5 +
 5 files changed, 163 insertions(+), 11 deletions(-)
 create mode 100755 tools/testing/selftests/net/mptcp/mptcp_connect_zerocopy.sh

-- 
2.53.0

Hi Geliang,

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): Unstable: 1 failed test(s): selftest_mptcp_connect_zerocopy ⚠️ 
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- 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/29154277861

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/f0a2581ba929
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1125835


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)