Re: [PATCH 5/6] tcp: Don't try to transmit right after the peer shrank the window to zero

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

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On Mon, Aug 18, 2025 at 07:06:29PM +0200, Stefano Brivio wrote:
> On Mon, 18 Aug 2025 12:56:09 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Fri, Aug 15, 2025 at 06:10:41PM +0200, Stefano Brivio wrote:
> > > If the peer shrinks the window to zero, we'll skip storing the new
> > > window, as a convenient way to cause window probes (which exceed any
> > > zero-sized window, strictly speaking) if we don't get window updates
> > > in a while.  
> > 
> > Strictly speaking, not storing the new zero window feels slightly
> > wrong to me - I wonder if it would be more correct to store the zero
> > window, but still send window probes as a special case.
> 
> Yeah, it's wrong but it's not really obvious what value we should use
> at that point. With a recent kernel version, there would be no problem
> storing 0, waiting for a window update which will come soon after, and
> reserve the special case I already added (1 as window size) for
> zero-window probes, triggered via timer.
> 
> But without 8c670bdfa58e ("tcp: correct handling of extreme memory
> squeeze"), and with e2142825c120 ("net: tcp: send zero-window ACK when
> no memory"), the window update never comes, so we would need to wait
> for the timer to expire before we send an explicit zero-window probe,
> which takes a while (two seconds).

Right.  I guess what I had in mind was to store the zero window, but
also set up a new timer to send a new window-probe in a more
reasonable timeframe.

> With this trick, instead, we keep the latest advertised value, which
> is actually the latest correct value, and retry sending what we were
> originally sending as soon as we have more data from the socket, which
> would becomes available quite soon in all the practical cases where the
> kernel shrinks the window to zero.

Ah, right.  So, IIUC a part of the issue is that at this point if we
see a zero window we don't know if it's because *we* filled the window
in the normal way - and so should wait for an update - or because the
peer slammed the window shut - in which case we need to reprobe.  I
mean, I guess we could set a flag for that earlier, but that's fairly
awkward.

At least for the time being, I think the current approach of keeping
the previous window is preferable.

> Maybe we could store zero, and use WINDOW_DEFAULT (14600 bytes) on
> socket activity. The RFC simply says we need to send zero-window
> probes at some point, not that we should assume a broken receiver,
> but unfortunately between e2142825c120 and 8c670bdfa58e it's like that.

I think the existing behaviour of keeping the previous window is
preferable.  At least for the specific case of a transient memory
error, that seems more likely to be right.  Maybe?  Unless we think we
should slow down in the hopes of not exhausting the peer's memory again.

-- 
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

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