A subflow closed after an error is never re-created, even once the path
is back (#440).

When TCP closes a subflow we created because of an error, the PM opens
a new one to the same addresses, and if it fails, the same socket is
used for the next try. A new sysctl, net.mptcp.subflow_retries, sets the
max tries.

Open question: an ICMP unreachable aborts the SYN right away, so the
tries can be used up in a few RTTs. Adding a delay would need a timer
again. Is that acceptable, or is there a better way?

Changes in v3 (Sashiko and CI reviews):
 - no retry after MP_RST without the T flag, local abort or EBADMSG
 - no retry on a removed endpoint or address
 - count the tries from scratch only after a minute of uptime
 - use only request_bkup for the backup flag
 - reused socket: SUB_CLOSED event, MIB counter and BPF pre_connect
 - lock_sock_nested() when reconnecting
 - patch 1: don't wait forever on EAGAIN
 - patch 3: check the subflow address, create the files before the
   background job, bigger file for slow debug kernels
 - wrap lines over 80 columns

Changes in v2:
 - use the subflow sockets, no list or timer
 - sysctl is now the max number of tries
 - code moved to pm_kernel.c
 - Fixes tag on patch 1
 - one selftest with netfilter
 - Assisted-by on all patches

v2: https://lore.kernel.org/mptcp/20261013025653.523755-1-anilkaushikwireless@gmail.com/
v1: https://lore.kernel.org/mptcp/20261012171740.464517-1-anilkaushikwireless@gmail.com/

Anil Kaushik (3):
  selftests: mptcp: connect: retry write() on EAGAIN
  mptcp: pm: kernel: retry subflows closed after an error
  selftests: mptcp: join: validate subflow retry after link failure

 Documentation/networking/mptcp-sysctl.rst     |  18 +++
 net/mptcp/ctrl.c                              |  13 ++
 net/mptcp/mib.c                               |   1 +
 net/mptcp/mib.h                               |   1 +
 net/mptcp/pm.c                                |   8 +
 net/mptcp/pm_kernel.c                         | 145 ++++++++++++++++++
 net/mptcp/protocol.c                          |   9 ++
 net/mptcp/protocol.h                          |  24 +++
 net/mptcp/subflow.c                           |  63 ++++++++
 .../selftests/net/mptcp/mptcp_connect.c       |  16 ++
 .../testing/selftests/net/mptcp/mptcp_join.sh | 106 +++++++++++++
 11 files changed, 404 insertions(+)

-- 
2.25.1

Hi Anil,

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): 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! ✅
- Perf: Success! ✅
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/38583978482

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/06eef3d66a29
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1185663


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)