netdev - Re: [PATCH net-next 09/15] net: increase MAX_SKB

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Message-ID: <CANn89iL3_6Vkj6Gq8vuMmC=pbAS+Zbe4hFaXgSEpyKhzfgh+dQ@mail.gmail.com>
Date:   Thu, 3 Feb 2022 11:20:39 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 09/15] net: increase MAX_SKB_FRAGS

On Thu, Feb 3, 2022 at 11:18 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 3 Feb 2022 09:56:42 -0800 Alexander Duyck wrote:
> > > I could make  MAX_SKB_FRAGS a config option, and default to 17, until
> > > all drivers have been fixed.
> > >
> > > Alternative is that I remove this patch from the series and we apply
> > > it to Google production kernels,
> > > as we did before.
> >
> > A config option would probably be preferred. The big issue as I see it
> > is that changing MAX_SKB_FRAGS is going to have ripples throughout the
> > ecosystem as the shared info size will be increasing and the queueing
> > behavior for most drivers will be modified as a result.
>
> I'd vote for making the change and dealing with the fall out. Unlikely
> many people would turn this knob otherwise and it's a major difference.
> Better not to fork the characteristics of the stack, IMHO.

Another issue with CONFIG_ options is that they are integer.

Trying the following did not work

#define MAX_SKB_FRAGS  ((unsigned long)CONFIG_MAX_SKB_FRAGS)

Because in some places we have

#if    (   MAX_SKB_FRAGS > ...)

(MAX_SKB_FRAGS is UL currently, making it an integer might cause some
signed/unsigned operations buggy)