Hi, this series ensures that rcv_wnd and window_clamp do not exceed the maximum window size representable for the connection's window scale factor.
This is most visible when TCP window scaling is not used for a connection. In that case, the advertised window is limited to 65535 bytes, but rcv_wnd or window_clamp can still grow beyond 65535 when large receive buffers are used. The resulting mismatch breaks calculations that depend on the advertised window, such as the ACK decision in __tcp_ack_snd_check(), and can prevent immediate ACKs. Similar effects may also occur when window scaling is in use, e.g. if the application dynamically adjusts SO_RCVBUF in unusual ways or when the rmem sysctl parameters change during a connection’s lifetime. Summary: - Patch 1 keeps rcv_wnd capped by the (window scale-limited) window_clamp at connection start. - Patch 3 and 6 ensure that window_clamp is limited to the representable window when it is updated. - The other patches add packetdrill tests to verify the new behavior. A simple iperf test on a virtme-ng VM (Intel i5-7500, 4 cores, loopback) shows a noticeable improvement with window scaling disabled: Fixed receive buffer: - sysctl net.ipv4.tcp_window_scaling=0 - Server: iperf -l 256K -w 256K -s - Client: iperf -l 256K -w 256K -c 127.0.0.1 -t 30 - net-next: ~47 Gbit/sec - with this series: ~62 Gbit/sec Receive buffer autotuning (net.ipv4.tcp_rmem = 4096 131072 7813888): - sysctl net.ipv4.tcp_window_scaling=0 - Server: iperf -s - Client: iperf -c 127.0.0.1 -t 30 - net-next: ~48 Gbit/sec - with this series: ~60 Gbit/sec Signed-off-by: Simon Baatz <[email protected]> --- Simon Baatz (7): tcp: keep rcv_wnd/rcv_ssthresh clamped by window_clamp if no scaling in use selftests/net: packetdrill: verify non-scaled rcv_wnd initialization tcp: Ensure window_clamp is limited to representable window selftests/net: packetdrill: add tcp_rcv_wnd_snd_ack_no_scaling.pkt selftests/net: packetdrill: add TCP_WINDOW_CLAMP test tcp: use tcp_set_window_clamp() for SO_RCVLOWAT selftests/net: packetdrill: add test for SO_RCVLOWAT window clamp net/ipv4/tcp.c | 6 +++- net/ipv4/tcp_input.c | 13 ++++++-- net/ipv4/tcp_minisocks.c | 8 +++-- net/ipv4/tcp_output.c | 5 +-- .../net/packetdrill/tcp_rcv_sockopt_lowat.pkt | 24 ++++++++++++++ .../net/packetdrill/tcp_rcv_sockopt_wnd_clamp.pkt | 28 ++++++++++++++++ .../packetdrill/tcp_rcv_wnd_active_no_scaling.pkt | 27 ++++++++++++++++ .../tcp_rcv_wnd_active_peer_no_scaling.pkt | 26 +++++++++++++++ .../packetdrill/tcp_rcv_wnd_passive_no_scaling.pkt | 30 ++++++++++++++++++ .../tcp_rcv_wnd_passive_peer_no_scaling.pkt | 29 +++++++++++++++++ .../packetdrill/tcp_rcv_wnd_snd_ack_no_scaling.pkt | 37 ++++++++++++++++++++++ 11 files changed, 225 insertions(+), 8 deletions(-) --- base-commit: b3e69fc3196fc421e26196e7792f17b0463edc6f change-id: 20260402-tcp_rcv_exact_clamp_and_wnd-427d853e7491 Best regards, -- Simon Baatz <[email protected]>
