Re: [PATCH] tcp: Don't discard window information on keep-alive segments

[Stefano Brivio <sbrivio@redhat.com>]

On Wed, 12 Feb 2025 11:42:54 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Tue, Feb 11, 2025 at 08:50:51PM +0100, Stefano Brivio wrote:
> > It looks like a detail, but it's critical if we're dealing with
> > somebody, such as near-future self, using TCP_REPAIR to migrate TCP
> > connections in the guest or container.
> > 
> > The last packet sent from the 'source' process/guest/container
> > typically reports a small window, or zero, because the guest/container
> > hadn't been draining it for a while.
> > 
> > The next packet, appearing as the target sets TCP_REPAIR_OFF on the
> > migrated socket, is a keep-alive (also called "window probe" in CRIU
> > or TCP_REPAIR-related code), and it comes with an updated window
> > value, reflecting the pre-migration "regular" value.
> > 
> > If we ignore it, it might take a while/forever before we realise we
> > can actually restart sending.
> > 
> > Fixes: 238c69f9af45 ("tcp: Acknowledge keep-alive segments, ignore them for the rest")
> > Signed-off-by: Stefano Brivio <sbrivio@redhat.com>  
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> 
> Although...
> 
> > ---
> >  tcp.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index af6bd95..2addf4a 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -1664,8 +1664,10 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  			tcp_send_flag(c, conn, ACK);
> >  			tcp_timer_ctl(c, conn);
> >  
> > -			if (p->count == 1)
> > +			if (p->count == 1) {  
> 
> ... not really this patch, but this condition seems wrong to me.  IIUC
> it's attempting to detect the last packet in the batch, which isn't
> necessarily the same thing as the _only_ packet in the batch.

No, not really, I just want to select one-packet batches on purpose. If
a keep-alive is part of a batch 1. it's not a keep-alive and 2. it
would probably need a more complicated handling which I hadn't really
time to think about.

See previous discussion on this:

  https://archives.passt.top/passt-dev/Zz01CDMNyFN-Ze68@zatzit

> Admittedly, it probably will be for a keep-alive, but I'm having a
> hard time convincing myself it absolutely has to be.

It is, because it makes no sense to batch keep-alives...

> Should this maybe be (i + 1 == p->count) instead?

-- 
Stefano