Re: [PATCH v4 3/8] tcp: Rewind sequence when guest shrinks window to zero

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

[-- Attachment #1: Type: text/plain, Size: 4872 bytes --]

On Wed, Sep 10, 2025 at 08:37:48AM +0200, Stefano Brivio wrote:
> On Wed, 10 Sep 2025 12:20:19 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Sep 09, 2025 at 08:16:50PM +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.
> > > 
> > > We need to do that both in tcp_tap_window_update(), where we already
> > > check for zero-window updates, as well as in tcp_data_from_tap(),
> > > because we might get one of those updates in a batch of packets that
> > > also contains a non-zero window update.
> > > 
> > > Suggested-by: Jon Maloy <jmaloy@redhat.com>
> > > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> > 
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> > 
> > Though a couple of documentation nits below.
> > 
> > > ---
> > >  tcp.c | 34 +++++++++++++++++++++++++---------
> > >  1 file changed, 25 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/tcp.c b/tcp.c
> > > index 86e08f1..12d42e0 100644
> > > --- a/tcp.c
> > > +++ b/tcp.c
> > > @@ -1268,19 +1268,25 @@ 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.  
> > 
> > As noted earlier "will ever be acknowledged" might be a bit
> > misleading.  Maybe "no data beyond the window will be acknowledged
> > until it is retransmitted".
> 
> Sorry, I actually fixed the patch after your same comment on v3 and
> then for some reason I threw away the changes together with some of
> your Reviewed-by: tags. Fixed as we discussed on v3 now.

Ah, right.  It's happened to all of us.

> 
> > >  	 */
> > > -	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);
> > >  
> > > @@ -1709,7 +1715,8 @@ 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;
> > >  			}
> > >  
> > > @@ -1728,6 +1735,15 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> > >  				       ack_seq == max_ack_seq &&
> > >  				       ntohs(th->window) == max_ack_seq_wnd;
> > >  
> > > +				/* See tcp_tap_window_update() for details. On
> > > +				 * top of that, we also need to check here if a
> > > +				 * zero-window update is contained in a batch of
> > > +				 * packets that includes a non-zero window as
> > > +				 * well.  
> > 
> > I'm not 100% convinced of this reasoning.  But at worst this should
> > result in some unnecessary but mostly harmless retransmits, and it
> > seems to fix the problem empirically, so I'm not suggesting changing
> > it at this time.
> 
> Right, strictly speaking, it *might* be that the peer didn't actually
> throw data away, which should be indicated by the fact that the same
> data is acknowledged in another segment of this same batch.
> 
> But handling that without making this part unreasonably complicated
> implies a refactor of this whole thing: we should extract a function
> dealing with sequences and perhaps another one with window updates from
> here.
> 
> I plan to file a ticket or two with stuff that emerged from this series.

Ok, sounds good.

-- 
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 --]