netdev - Re: [PATCH]: Fix sk_buff page offsets and lengths.

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Date:	Tue, 31 Jul 2007 10:52:15 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, sfr@...b.auug.org.au,
	shemminger@...ux-foundation.org
Subject: Re: [PATCH]: Fix sk_buff page offsets and lengths.

On Mon, 30 Jul 2007 18:50:28 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> 
> Stephen Rothwell pointed out to me that the skb_frag_struct
> is broken on platforms using 64K or larger page sizes, it
> even generates warnings when (for example) the myri10ge driver
> tries to assign PAGE_SIZE into frag->size.
> 
> I've thus increased page offset and size to __u32 in the patch below.
> 
> I made this change much to even my own chagrin, but this is the
> most direct fix and the ifdefs we could put here are both ugly
> and also not something that we do with struct scatterlist so
> no reason to do it in a place like this.
> 
> Actually, the cost on 64-bit is zero because there existed 4 bytes of
> alignment padding for skb_frag_struct because of the page pointer.
> On 32-bit the cost is up to 64-bytes :-/
> 
> Stephen, this opens up the doors a bit for the scatterlist work
> you wanted to do in sk_buff.
> 

Ouch... 

sizeof(struct skb_shared_info) is enlarged by 18*4 bytes on i386, a litle bit more than 64 bytes :(

I understand ifdefs are ugly, but in the common case (PAGE_SIZE<64K), this change seems very unfortunate.

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