Re: [PATCH 3/6] tcp: Rewind sequence when guest shrinks window to zero

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 3902 bytes --]

On Fri, Aug 15, 2025 at 06:10:39PM +0200, Stefano Brivio wrote:
> A window shrunk to zero means by definition that anything else that
> might be in flight is now out of window. Restart from the currently
> acknowledged sequence.
> 
> Suggested-by: Jon Maloy <jmaloy@redhat.com>
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tcp.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 1402ca2..dda0a2e 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1257,19 +1257,25 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn,
>  
>  /**
>   * tcp_tap_window_update() - Process an updated window from tap side
> + * @c:		Execution context
>   * @conn:	Connection pointer
>   * @wnd:	Window value, host order, unscaled
>   */
> -static void tcp_tap_window_update(struct tcp_tap_conn *conn, unsigned wnd)
> +static void tcp_tap_window_update(const struct ctx *c,
> +				  struct tcp_tap_conn *conn, unsigned wnd)
>  {
>  	wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap);
>  
>  	/* Work-around for bug introduced in peer kernel code, commit
> -	 * e2142825c120 ("net: tcp: send zero-window ACK when no memory").
> -	 * We don't update if window shrank to zero.
> +	 * e2142825c120 ("net: tcp: send zero-window ACK when no memory"): don't
> +	 * update the window if it shrank to zero, so that we'll eventually
> +	 * retry to send data, but rewind the sequence as that obviously implies
> +	 * that no data beyond the updated window will ever be acknowledged.
>  	 */
> -	if (!wnd && SEQ_LT(conn->seq_ack_from_tap, conn->seq_to_tap))
> +	if (!wnd && SEQ_LT(conn->seq_ack_from_tap, conn->seq_to_tap)) {
> +		tcp_rewind_seq(c, conn);
>  		return;
> +	}
>  
>  	conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);
>  
> @@ -1694,7 +1700,8 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  			tcp_timer_ctl(c, conn);
>  
>  			if (p->count == 1) {
> -				tcp_tap_window_update(conn, ntohs(th->window));
> +				tcp_tap_window_update(c, conn,
> +						      ntohs(th->window));
>  				return 1;
>  			}
>  
> @@ -1772,7 +1779,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  	if (ack && !tcp_sock_consume(conn, max_ack_seq))
>  		tcp_update_seqack_from_tap(c, conn, max_ack_seq);
>  
> -	tcp_tap_window_update(conn, max_ack_seq_wnd);
> +	tcp_tap_window_update(c, conn, max_ack_seq_wnd);
>  
>  	if (retr) {
>  		flow_trace(conn,
> @@ -1861,7 +1868,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c,
>  				      const struct tcphdr *th,
>  				      const char *opts, size_t optlen)
>  {
> -	tcp_tap_window_update(conn, ntohs(th->window));
> +	tcp_tap_window_update(c, conn, ntohs(th->window));
>  	tcp_get_tap_ws(conn, opts, optlen);
>  
>  	/* First value is not scaled */
> @@ -2059,7 +2066,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  		if (!th->ack)
>  			goto reset;
>  
> -		tcp_tap_window_update(conn, ntohs(th->window));
> +		tcp_tap_window_update(c, conn, ntohs(th->window));
>  
>  		tcp_data_from_sock(c, conn);
>  
> @@ -2071,7 +2078,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  	if (conn->events & TAP_FIN_RCVD) {
>  		tcp_sock_consume(conn, ntohl(th->ack_seq));
>  		tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
> -		tcp_tap_window_update(conn, ntohs(th->window));
> +		tcp_tap_window_update(c, conn, ntohs(th->window));
>  		tcp_data_from_sock(c, conn);
>  
>  		if (conn->events & SOCK_FIN_RCVD &&

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]