Re: [net-next 2/2] tcp: correct handling of extreme menory squeeze

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  3 Apr 2024 18:58:33 -0400
Jon Maloy <jmaloy@redhat.com> wrote:

> Testing of the previous commit ("tcp: add support for SO_PEEK_OFF")
> in this series along with the pasta protocol splicer revealed a bug in
> the way tcp handles window advertising during extreme memory squeeze
> situations.
> 
> The excerpt of the below logging session shows what is happeing:
> 
> [5201<->54494]:     ==== Activating log @ tcp_select_window()/268 ====
> [5201<->54494]:     (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE
> [5201<->54494]:   tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0
> [5201<->54494]:   ADVERTISING WINDOW SIZE 0
> [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> 
> [5201<->54494]: tcp_recvmsg_locked(->)
> [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> [5201<->54494]:     NOT calling tcp_send_ack()
> [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83
> 
> [...]
> 
> [5201<->54494]: tcp_recvmsg_locked(->)
> [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> [5201<->54494]:     NOT calling tcp_send_ack()
> [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1
> 
> [5201<->54494]: tcp_recvmsg_locked(->)
> [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]:     (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0
> [5201<->54494]:     NOT calling tcp_send_ack()
> [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0
> 
> [5201<->54494]: tcp_recvmsg_locked(->)
> [5201<->54494]:   __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]:     NOT calling tcp_send_ack()
> [5201<->54494]:   __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354
> [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0
> 
> We can see that although we are adverising a window size of zero,
> tp->rcv_wnd is not updated accordingly. This leads to a discrepancy
> between this side's and the peer's view of the current window size.
> - The peer thinks the window is zero, and stops sending.
> - This side ends up in a cycle where it repeatedly caclulates a new
>   window size it finds too small to advertise.
> 
> Hence no messages are received, and no acknowledges are sent, and
> the situation remains locked even after the last queued receive buffer
> has been consumed.
> 
> We fix this by setting tp->rcv_wnd to 0 before we return from the
> function tcp_select_window() in this particular case.
> Further testing shows that the connection recovers neatly from the
> squeeze situation, and traffic can continue indefinitely.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  net/ipv4/tcp_output.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index e3167ad96567..5803fd402708 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -264,8 +264,11 @@ static u16 tcp_select_window(struct sock *sk)
>  	 * are out of memory. The window is temporary, so we don't store
>  	 * it on the socket.

One nit: now that you do store it on the socket, you should probably
change this comment as well.

>  	 */
> -	if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM))
> +	if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) {
> +		tp->rcv_wnd = 0;
> +		tp->rcv_wup = tp->rcv_nxt;

...I'm wondering if you should set 'pred_flags' to 0, as it's done at
the end of the function for other cases where the window is advertised
as zero.

At least according to the comment to tcp_rcv_established() it looks
like it's needed:

 *      - A zero window was announced from us - zero window probing
 *        is only handled properly in the slow path.

>  		return 0;
> +	}
>  
>  	cur_win = tcp_receive_window(tp);
>  	new_win = __tcp_select_window(sk);

The rest, including 1/2, looks good to me.

-- 
Stefano