netdev - Re: [PATCH] solos-pci: Increase headroom on received packets

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Date:	Wed, 16 Sep 2015 03:53:33 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	netdev@...r.kernel.org, simon@...e.lp0.eu, guy@...verse.com.au,
	linux-atm-general@...ts.sourceforge.net
Subject: Re: [PATCH] solos-pci: Increase headroom on received packets

On Wed, 2015-09-16 at 11:25 +0100, David Woodhouse wrote:
> A comment in include/linux/skbuff.h says that:
> 
>  * Various parts of the networking layer expect at least 32 bytes of
>  * headroom, you should not reduce this.
> 
> This was demonstrated by a panic when handling fragmented IPv6 packets:
> http://marc.info/?l=linux-netdev&m=144236093519172&w=2
> 
> It's not entirely clear if that comment is still valid — and if it is,
> perhaps netif_rx() ought to be enforcing it with a warning.
> 
> But either way, it is rather stupid from a performance point of view
> for us to be receiving packets into a buffer which doesn't have enough
> room to prepend an Ethernet header — it means that *every* incoming
> packet is going to be need to be reallocated. So let's fix that.
> 
> Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> --- 
> Tested in the DMA code path; I don't believe the DMA-capable devices
> can still be used in MMIO mode. Simon, Guy, would you be able to test
> the MMIO version?

You should use netdev_alloc_skb() : This helper is better for rx skbs,
as it allows for better packing of frames in GRO or TCP stack.

Also netdev_alloc_skb_ip_align() might handle the NET_IP_ALIGN stuff
for arches that care.


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