netdev - Re: [PATCH net-next 1/2] tcp: do not set a zero size receive buffer

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Message-ID: <CANn89i+LYOmZXerPTfQDprSGo=AZNhuXt52qZ6vnXnNf6DyqJA@mail.gmail.com>
Date: Mon, 21 Jul 2025 09:50:14 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, 
	Neal Cardwell <ncardwell@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>, 
	"David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Simon Horman <horms@...nel.org>, Matthieu Baerts <matttbe@...nel.org>
Subject: Re: [PATCH net-next 1/2] tcp: do not set a zero size receive buffer

On Mon, Jul 21, 2025 at 8:27 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 21 Jul 2025 16:56:06 +0200 Paolo Abeni wrote:
> > >> I *think* that catching only the !sk_rmem_alloc case would avoid the
> > >> stall, but I think it's a bit 'late'.
> > >
> > > A packetdrill test here would help understanding your concern.
> >
> > I fear like a complete working script would take a lot of time, let me
> > try to sketch just the relevant part:
> >
> > # receiver state is:
> > # rmem=110592 rcvbuf=174650 scaling_ratio=253 rwin=63232
> > # no OoO data, no memory pressure,
> >
> > # the incoming packet is in sequence
> > +0 > P. 109297:172528(63232) ack 1
> >
> > With just the 0 rmem check in tcp_prune_queue(), such function will
> > still invoke tcp_clamp_window() that will shrink the receive buffer to
> > 110592.
> > tcp_collapse() can't make enough room and the incoming packet will be
> > dropped. I think we should instead accept such packet.
> >
> > Side note: the above data are taken from an actual reproduction of the issue
> >
> > Please LMK if the above clarifies a bit my doubt or if a full pktdrill
> > is needed.
>
> Not the first time we stumble on packetdrills for scaling ratio.
> Solving it is probably outside the scope of this discussion but
> I wonder what would be the best way to do it. My go to is to
> integrate packetdrill with netdevsim and have an option for netdevsim
> to inflate truesize on demand. But perhaps there's a clever way we can
> force something like tap to give us the ratio we desire. Other ideas?

Adding a TUN option (ioctl) to be able to increase skb->truesize by a
given amount / percentage is doable I think.

Then in packetdrill we could add the ability to use this option for a
given scenario or packet.