netdev - Re: [PATCH net-next 1/2] net: Export skb_zerocopy() to zerocopy from one skb to another

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Message-ID: <20130524160106.GA30252@casper.infradead.org>
Date:	Fri, 24 May 2013 17:01:06 +0100
From:	Thomas Graf <tgraf@...g.ch>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, jesse@...ira.com, dev@...nvswitch.org
Subject: Re: [PATCH net-next 1/2] net: Export skb_zerocopy() to zerocopy from
 one skb to another

On 05/24/13 at 08:31am, Eric Dumazet wrote:
> Hmm. Big warning here : In the nfnetlink case, we specifically made sure
> that this path was taken only if skb->head_frag was set (all incoming
> packets have this property, but not outgoing ones yet)
> 
> This should be documented if we move it to net/core/skbuff.c, or we
> should add a BUG_ON()

Good point.

I guess we could ensure in skb_zerocopy() that if !head_frag then we
enforce hlen = max(hlen, skb_headlen(skb)) to always fall back to
skb_copy_bits() for !head_frag.

> I see your openvswitch code seems OK.
> 
> Maybe also put in net/core/skbuff.c the code dealing with  this stuff ?
> 
> +       if (!skb->head_frag ||
> +           skb_headlen(skb) < L1_CACHE_BYTES ||
> +           skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)
> +               hlen = skb_headlen(skb);
> +
> +       if (skb_has_frag_list(skb))
> +               hlen = skb->len;

I'll put this into skb_zerocopy_hdrlen()
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