Re: [PATCH net-next] mptcp: fix rcv buffer auto-tuning

[Eric Dumazet]

On Thu, Jul 20, 2023 at 8:48 PM Matthieu Baerts
<matthieu.baerts@xxxxxxxxxxxx> wrote:
>
> From: Paolo Abeni <pabeni@xxxxxxxxxx>
>
> The MPTCP code uses the assumption that the tcp_win_from_space() helper
> does not use any TCP-specific field, and thus works correctly operating
> on an MPTCP socket.
>
> The commit dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> broke such assumption, and as a consequence most MPTCP connections stall
> on zero-window event due to auto-tuning changing the rcv buffer size
> quite randomly.
>
> Address the issue syncing again the MPTCP auto-tuning code with the TCP
> one. To achieve that, factor out the windows size logic in socket
> independent helpers, and reuse them in mptcp_rcv_space_adjust(). The
> MPTCP level scaling_ratio is selected as the minimum one from the all
> the subflows, as a worst-case estimate.
>
> Fixes: dfa2f0483360 ("tcp: get rid of sysctl_tcp_adv_win_scale")
> Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
> Co-developed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
> ---
>  include/net/tcp.h    | 20 +++++++++++++++-----
>  net/mptcp/protocol.c | 15 +++++++--------
>  net/mptcp/protocol.h |  8 +++++++-
>  net/mptcp/subflow.c  |  2 +-
>  4 files changed, 30 insertions(+), 15 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index c5fb90079920..794642fbd724 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1430,22 +1430,32 @@ void tcp_select_initial_window(const struct sock *sk, int __space,
>                                __u32 *window_clamp, int wscale_ok,
>                                __u8 *rcv_wscale, __u32 init_rcv_wnd);
>
> -static inline int tcp_win_from_space(const struct sock *sk, int space)
> +static inline int __tcp_win_from_space(u8 scaling_ratio, int space)
>  {
> -       s64 scaled_space = (s64)space * tcp_sk(sk)->scaling_ratio;
> +       s64 scaled_space = (s64)space * scaling_ratio;
>
>         return scaled_space >> TCP_RMEM_TO_WIN_SCALE;
>  }
>
> -/* inverse of tcp_win_from_space() */
> -static inline int tcp_space_from_win(const struct sock *sk, int win)
> +static inline int tcp_win_from_space(const struct sock *sk, int space)

Maybe in a follow up patch we could change the prototype of this helper
to avoid future mis use :)

static inline int tcp_win_from_space(const struct tcp_sock *tp, int space)
{
}

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>


> +{
> +       return __tcp_win_from_space(tcp_sk(sk)->scaling_ratio, space);
> +}
> +
> +/* inverse of __tcp_win_from_space() */
> +static inline int __tcp_space_from_win(u8 scaling_ratio, int win)
>  {
>         u64 val = (u64)win << TCP_RMEM_TO_WIN_SCALE;
>
> -       do_div(val, tcp_sk(sk)->scaling_ratio);
> +       do_div(val, scaling_ratio);
>         return val;
>  }
>
> +static inline int tcp_space_from_win(const struct sock *sk, int win

Same remark here.

Thanks for the fix !