netdev - Re: [PATCH net] tcp: fix mid stream window clamp.

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Date: Thu, 23 Nov 2023 19:16:12 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>, Neal Cardwell
 <ncardwell@...gle.com>,  Wei Wang <weiwan@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, David
 Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Neil Spring
 <ntspring@...com>,  David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH net] tcp: fix mid stream window clamp.

On Thu, 2023-11-23 at 18:10 +0100, Eric Dumazet wrote:
> CC Neal and Wei
> 
> On Thu, Nov 23, 2023 at 4:25 PM Paolo Abeni <pabeni@...hat.com> wrote:
> > 
> > After the blamed commit below, if the user-space application performs
> > window clamping when tp->rcv_wnd is 0, the TCP socket will never be
> > able to announce a non 0 receive window, even after completely emptying
> > the receive buffer and re-setting the window clamp to higher values.
> > 
> > Refactor tcp_set_window_clamp() to address the issue: when the user
> > decreases the current clamp value, set rcv_ssthresh according to the
> > same logic used at buffer initialization time.
> > When increasing the clamp value, give the rcv_ssthresh a chance to grow
> > according to previously implemented heuristic.
> > 
> > Fixes: 3aa7857fe1d7 ("tcp: enable mid stream window clamp")
> > Reported-by: David Gibson <david@...son.dropbear.id.au>
> > Reported-by: Stefano Brivio <sbrivio@...hat.com>
> > Reviewed-by: Stefano Brivio <sbrivio@...hat.com>
> > Tested-by: Stefano Brivio <sbrivio@...hat.com>
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> >  net/ipv4/tcp.c | 19 ++++++++++++++++---
> >  1 file changed, 16 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 53bcc17c91e4..1a9b9064e080 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -3368,9 +3368,22 @@ int tcp_set_window_clamp(struct sock *sk, int val)
> >                         return -EINVAL;
> >                 tp->window_clamp = 0;
> >         } else {
> > -               tp->window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
> > -                       SOCK_MIN_RCVBUF / 2 : val;
> > -               tp->rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
> > +               u32 new_rcv_ssthresh, old_window_clamp = tp->window_clamp;
> > +               u32 new_window_clamp = val < SOCK_MIN_RCVBUF / 2 ?
> > +                                               SOCK_MIN_RCVBUF / 2 : val;
> > +
> > +               if (new_window_clamp == old_window_clamp)
> > +                       return 0;
> > +
> > +               tp->window_clamp = new_window_clamp;
> > +               if (new_window_clamp < old_window_clamp) {
> > +                       tp->rcv_ssthresh = min(tp->rcv_ssthresh,
> > +                                              new_window_clamp);
> > +               } else {
> > +                       new_rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
> > +                       tp->rcv_ssthresh = max(new_rcv_ssthresh,
> > +                                              tp->rcv_ssthresh);
> > +               }
> >         }
> >         return 0;
> >  }
> 
> It seems there is no provision for SO_RESERVE_MEM

Indeed I did take that in account.

> I wonder if tcp_adjust_rcv_ssthresh()  could help here ?

I don't know how to fit it into the above. tcp_adjust_rcv_ssthresh()
tends to shrink rcv_ssthresh to low values when no memory is reserved. 

Dealing directly with SO_RESERVE_MEM when shrinking the threshold feels
easier to me, something alike:

               if (new_window_clamp == old_window_clamp)
                       return 0;

               tp->window_clamp = new_window_clamp;
               if (new_window_clamp < old_window_clamp) {
			int unused_mem = sk_unused_reserved_mem(sk);

			tp->rcv_ssthresh = min(tp->rcv_ssthresh,
                                               new_window_clamp);

			if (unused_mem)
				tp->rcv_ssthresh = max_t(u32, tp->rcv_ssthresh,
					tcp_win_from_space(sk, unused_mem));
               } else {
			new_rcv_ssthresh = min(tp->rcv_wnd, tp->window_clamp);
			tp->rcv_ssthresh = max(new_rcv_ssthresh,
                                              tp->rcv_ssthresh);
               }

Possibly the bits shared with tcp_adjust_rcv_ssthresh() could be
factored out in a common helper.

> Have you considered reverting  3aa7857fe1d7 ("tcp: enable mid stream
> window clamp") ?


That would work, too and will be simpler.

The issue at hand was noted with an application that really wants to
limit the announced window:

https://gitlab.com/dgibson/passt

I guess touching rcv_ssthresh would be a bit more effective. 

Not much more in the end, as both window_clamp and rcv_ssthresh can
later grow due to rcv buf auto-tune. Ideally we would like to prevent
tcp_rcv_space_adjust() from touching window_clamp after
TCP_WINDOW_CLAMP - but that is another matter/patch.

Thanks!

Paolo