netdev - Re: [PATCH net] net: allow small head cache usage with large MAX_SKB

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Message-ID: <CANn89i+us2jYB6ayce=8GuSKJjjyfH4xj=FvB9ykfMD3=Sp=tw@mail.gmail.com>
Date: Thu, 13 Feb 2025 14:44:04 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>
Subject: Re: [PATCH net] net: allow small head cache usage with large
 MAX_SKB_FRAGS values

On Wed, Feb 12, 2025 at 11:08 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 2/12/25 9:47 PM, Eric Dumazet wrote:
> > This patch still gives a warning if  MAX_TCP_HEADER < GRO_MAX_HEAD +
> > 64 (in my local build)
>
> Oops, I did not consider MAX_TCP_HEADER and GRO_MAX_HEAD could diverge.
>
> > Why not simply use SKB_WITH_OVERHEAD(SKB_SMALL_HEAD_CACHE_SIZE) , and
> > remove the 1024 value ?
>
> With CONFIG_MAX_SKB_FRAGS=17, SKB_SMALL_HEAD_CACHE_SIZE is considerably
> smaller than 1024, I feared decreasing such limit could re-introduce a
> variation of the issue addressed by commit 3226b158e67c ("net: avoid 32
> x truesize under-estimation for tiny skbs").
>
> Do you feel it would be safe?

As long as we are using kmalloc() for those, we are good I think.

With MAX_SKB_FRAGS=17, I have :

# grep small /proc/slabinfo
skbuff_small_head    276    391    704   23    4 : tunables    0    0
  0 : slabdata     17     17      0