linux-kernel - Re: [PATCH net] net: napi: Make napi_defer

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Message-ID: <ZtXv-ywidT58IyNj@LQ3V64L9R2.station>
Date: Mon, 2 Sep 2024 19:03:55 +0200
From: Joe Damato <jdamato@...tly.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, mkarsten@...terloo.ca, stable@...nel.org,
	Jakub Kicinski <kuba@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Paolo Abeni <pabeni@...hat.com>, Jonathan Corbet <corbet@....net>,
	Breno Leitao <leitao@...ian.org>,
	Johannes Berg <johannes.berg@...el.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: napi: Make napi_defer_irqs u32

On Mon, Sep 02, 2024 at 03:01:28PM +0200, Eric Dumazet wrote:
> On Sat, Aug 31, 2024 at 1:32 PM Joe Damato <jdamato@...tly.com> wrote:
> >
> > In commit 6f8b12d661d0 ("net: napi: add hard irqs deferral feature")
> > napi_defer_irqs was added to net_device and napi_defer_irqs_count was
> > added to napi_struct, both as type int.
> >
> > This value never goes below zero. Change the type for both from int to
> > u32, and add an overflow check to sysfs to limit the value to S32_MAX.
> >
> > Before this patch:
> >
> > $ sudo bash -c 'echo 2147483649 > /sys/class/net/eth4/napi_defer_hard_irqs'
> > $ cat /sys/class/net/eth4/napi_defer_hard_irqs
> > -2147483647
> >
> > After this patch:
> >
> > $ sudo bash -c 'echo 2147483649 > /sys/class/net/eth4/napi_defer_hard_irqs'
> > bash: line 0: echo: write error: Numerical result out of range
> >
> > Fixes: 6f8b12d661d0 ("net: napi: add hard irqs deferral feature")
> > Cc: stable@...nel.org
> > Cc: Eric Dumazet <edumazet@...gle.com>
> > Suggested-by: Jakub Kicinski <kuba@...nel.org>
> > Signed-off-by: Joe Damato <jdamato@...tly.com>
> > ---
> 
> I do not think this deserves a change to stable trees.
> 
> Signed or unsigned, what is the issue ?
> 
> Do you really need one extra bit ?
> 
> /sys/class/net/XXXXX/tx_queue_len has a similar behavior.

Sorry, Eric, I'm not following.

Are you asking me to allow u32_max for napi_defer_hard_irqs the same
way tx_queue_len does and avoid overflow that way:

include/linux/netdevice.h:      unsigned int            tx_queue_len;

dev_change_tx_queue_len(..., unsigned long new_len):

  if (new_len != (unsigned int)new_len)
          return -ERANGE;