netdev - [PATCH net-next v2 0/6] tcp: support preloading data on a listening socket

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Message-ID: <cover.1747826775.git.jgh@exim.org>
Date: Wed, 21 May 2025 12:44:59 +0100
From: Jeremy Harris <jgh@...m.org>
To: netdev@...r.kernel.org
Cc: linux-api@...r.kernel.org,
	edumazet@...gle.com,
	ncardwell@...gle.com,
	Jeremy Harris <jgh@...m.org>
Subject: [PATCH net-next v2 0/6] tcp: support preloading data on a listening socket

v2 changes:
  - Split out the preload operation to a separate routine from
    tcp_sendmsg_locked() and restrict from looping over the supplied
    iovec

------
Support write to a listen TCP socket, for immediate
transmission on all later passive connection establishments
parented by the listen socket.

On a normal connection transmission of the data is triggered by the receipt
of the 3rd-ack. On a fastopen (with accepted cookie) connection the data
is sent in the synack packet.

The data preload is done using a sendmsg with a newly-defined flag
(MSG_PRELOAD); the amount of data limited to a single linear sk_buff.
Note that this definition is the last-but-two bit available if "int"
is 32 bits.

Intent: lower latency for server-first protocols using TCP.
  Known cases of this use are SMTP and MySQL.

  Measurements:
    Packet capture (laptop, loopback, TFO requeste) for initial SYN to first
    client data packet (5 samples):

    - baseline   TFO-C      1064 1470 1455 1547 1595  usec
    - patched    non-TFO     140  150  159  144  153  usec
    - patched    TFO-C       142  149  149  125  125  usec

  Out of scope:
  - Client-first protocols
  - TLS-on-connect

Testing:

A) packetdrill scripts for
   - normal non-TFO
   - normal TFO
   - synack lost
   - 3rd-ack acks only the SYN
   - 3rd-ack acks partial data
     (NB: packetdrill can only check the data size, not actual content)

B) Application use, running the application testsuite
   and manual check of specific cases via packet capture

C) Daily-driver laptop use (not expected to trigger the feature;
   only regression-test)

D) KASAN/syzkaller
   - enable_syscalls "socket$inet_tcp", "listen", "sendmsg", "accept",
      "read", "write", "close", "syz_emit_ethernet", "syz_extract_tcp_res"
   - the coverage seems rather limited; the sendmsg onto a listen socket
     is there, but I am not convinced actual TCP connections are being
     excercised.  tcp_minisocks.c is entirely uncovered.
   - A need for limiting iteration in the above sendmesg was found (RCU
     timeouts), hence v2, but no hint of locking problems.
     Eric: could you expand on your previous comment?  If it referred to
     the listening socket, tcp_sendmsg_locked() is called with the sk
     locked.

Jeremy Harris (6):
  tcp: support writing to a socket in listening state
  tcp: copy write-data from listen socket to accept child socket
  tcp: fastopen: add write-data to fastopen synack packet
  tcp: transmit any pending data on receipt of 3rd-ack
  tcp: fastopen: retransmit data when only the SYN of a synack-with-data
    is acked
  tcp: fastopen: extend retransmit-queue trimming to handle linear
    sk_buff

 include/linux/socket.h                        |   1 +
 net/ipv4/tcp.c                                | 115 ++++++++++++++++++
 net/ipv4/tcp_fastopen.c                       |   3 +-
 net/ipv4/tcp_input.c                          |  15 ++-
 net/ipv4/tcp_ipv4.c                           |   4 +-
 net/ipv4/tcp_minisocks.c                      |  58 ++++++++-
 net/ipv4/tcp_output.c                         |  50 +++++++-
 .../perf/trace/beauty/include/linux/socket.h  |   1 +
 tools/perf/trace/beauty/msg_flags.c           |   3 +
 9 files changed, 237 insertions(+), 13 deletions(-)


base-commit: f685204c57e87d2a88b159c7525426d70ee745c9
-- 
2.49.0