netdev - Re: [RFC] truesize lies

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Message-ID: <1318197320.21116.11.camel@edumazet-laptop>
Date:	Sun, 09 Oct 2011 23:55:20 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] truesize lies

Le dimanche 09 octobre 2011 à 17:27 -0400, David Miller a écrit :
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Sun, 09 Oct 2011 21:02:28 +0200
> 
> > I guess most driver writers adjust truesize to please TCP stack and get
> > nice performance numbers.
> > 
> > skb->truesize = frame_len + sizeof(sk_buff);
> > 
> > What should we do exactly ?
> 
> We should pick a specification that accounts for all the metadata as well
> as unused data areas properly, and enforce this in netif_receive_skb().
> 
> That will get the inaccurate cases fixed real fast.
> 
> We've had similar problems in the past and dealt with it in the same
> exact way.


Some drivers splits a page in two (or more) pieces, so we cant know what
was really reserved by a driver for paged skbs.

Only thing we could enforce at the moment is the proper accounting for
skb head :

WARN_ON(skb->truesize < (skb_end_pointer(skb) - skb->head) + sizeof(sk_buff) + skb->data_len);



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