netdev - Re: [Intel-wired-lan] [PATCH net-next 1/2] stmmac: xsk: fix underflow of budget in zerocopy mode

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Message-ID: <CAL+tcoC5KnTuWKxKcUqFGh-nBSF+X+RWzr=RkkK86+jY1Q20Kw@mail.gmail.com>
Date: Tue, 22 Jul 2025 23:53:03 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Paul Menzel <pmenzel@...gen.mpg.de>, anthony.l.nguyen@...el.com, 
	przemyslaw.kitszel@...el.com, andrew+netdev@...n.ch, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, bjorn@...nel.org, 
	magnus.karlsson@...el.com, maciej.fijalkowski@...el.com, 
	jonathan.lemon@...il.com, sdf@...ichev.me, ast@...nel.org, 
	daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com, 
	mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com, 
	linux-stm32@...md-mailman.stormreply.com, bpf@...r.kernel.org, 
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [Intel-wired-lan] [PATCH net-next 1/2] stmmac: xsk: fix underflow
 of budget in zerocopy mode

On Tue, Jul 22, 2025 at 10:16 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > Hi Paul,
> >
> > On Mon, Jul 21, 2025 at 4:56 PM Paul Menzel <pmenzel@...gen.mpg.de> wrote:
> > >
> > > Dear Jason,
> > >
> > >
> > > Thank you for your patch.
> >
> > Thanks for your quick response and review :)
> >
> > >
> > > Am 21.07.25 um 10:33 schrieb Jason Xing:
> > > > From: Jason Xing <kernelxing@...cent.com>
> > > >
> > > > The issue can happen when the budget number of descs are consumed. As
> > >
> > > Instead of “The issue”, I’d use “An underflow …”.
> >
> > Will change it.
> >
> > >
> > > > long as the budget is decreased to zero, it will again go into
> > > > while (budget-- > 0) statement and get decreased by one, so the
> > > > underflow issue can happen. It will lead to returning true whereas the
> > > > expected value should be false.
> > >
> > > What is “it”?
> >
> > It means 'underflow of budget' behavior.
>
> A technicality, but this is (negative) overflow.
>
> Underflow is a computation that results in a value that is too small
> to be represented by the given type.

Interesting. Thanks for sharing this with me:)

I just checked the wikipedia[1] that says " Underflow can in part be
regarded as negative overflow of the exponent of the floating-point
value.". I assume this rule can also be applied in this case? I'm
hesitant to send the v3 patch tomorrow with this 'negative overflow'
term included.

[1]: https://en.wikipedia.org/wiki/Arithmetic_underflow

Thanks,
Jason