Re: [RFC] allow skb->head to point/alias to first skb frag

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-04-26 at 05:18 -0400, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Thu, 26 Apr 2012 11:10:02 +0200
> 
> > The 'frag' would have a known size : 2048 bytes
> > 
> > But the end of it would be used by struct skb_shared_info
> > 
> > so data_len would be 0 in fact.
> > 
> > This would look like a regular linear skb.
> > 
> > Just a bit set in skb to say : Warning, skb->head was not kmalloced :
> > replace kfree(head) by put_page(...)
> > 
> > And this bit would be tested in GRO or tcp merge to 'upgrade' this
> > skb->head to proper page/frag
> 
> And what happens if this ends up in a piece of code which wants to
> append a page frag?
> 
> Or a piece of code which copies an SKB, including page frag parts?
> 
> All I'm saying is that the number of tests necessary to make this work
> properly might become prohibitive.

I'll cook a patch, I believe not so many parts assume skb->head can be
kmalloc()/kfree(). This should be contained in net/core/skbuff.c only.

Elsewhere we use skb->head as a pointer to memory. It will stay the
same.