netdev - Re: [PATCH net-next] ndisc: use netdev_alloc

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Message-ID: <1402516702.3645.471.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 11 Jun 2014 12:58:22 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] ndisc: use netdev_alloc_skb

On Wed, 2014-06-11 at 11:31 -0700, Stephen Hemminger wrote:
> IPv6 neighbor discovery should use netdev_alloc_skb which adds
> extra space for headroom. The old code would allocate packet w/o any
> additional headroom and force drivers to reallocate and copy the
> packet in some cases.
> 
> Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>
> 
> 
> --- a/net/ipv6/ndisc.c	2014-06-03 08:06:35.549942475 -0700
> +++ b/net/ipv6/ndisc.c	2014-06-11 08:54:31.729764741 -0700
> @@ -375,7 +375,7 @@ static struct sk_buff *ndisc_alloc_skb(s
>  	struct sock *sk = dev_net(dev)->ipv6.ndisc_sk;
>  	struct sk_buff *skb;
>  
> -	skb = alloc_skb(hlen + sizeof(struct ipv6hdr) + len + tlen, GFP_ATOMIC);
> +	skb = netdev_alloc_skb(dev, hlen + sizeof(struct ipv6hdr) + len + tlen);
>  	if (!skb) {

This looks strange.

netdev_alloc_skb() is mostly used for RX.

Perhaps you need to increase hlen with the needed room, because
NET_SKB_PAD might be only 32, and its a bit hidden.

ie use LL_RESERVED_SPACE_EXTRA() instead of LL_RESERVED_SPACE(), to make
clear what the intentions are.


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