Re: compound skb frag pages appearing in start_xmit

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

On Tue, 2012-10-09 at 15:54 +0200, Eric Dumazet wrote:
> On Tue, 2012-10-09 at 14:47 +0100, Ian Campbell wrote:
> > Hi Eric,
> > 
> 
> Hi Ian
> 
> > Sander has discovered an issue where xen-netback is given a compound
> > page as one of the skb frag pages to transmit. Currently netback can
> > only handle PAGE_SIZE'd frags and bugs out.
> > 
> > I suspect this is something to do with 69b08f62e174 "net: use bigger
> > pages in __netdev_alloc_frag", although perhaps not because it looks
> > like only tg3 uses it and Sander has an r8169. Also tg3 seems to only
> > call netdev_alloc_frag for sizes < PAGE_SIZE. I'm probably missing
> > something.
> 
> 
> Its not the commit you want ;)

Hmm, I take it back. It also can give you the same problem :

We use this allocator for rx path of drivers : 

 __netdev_alloc_skb() 

So its now absolutely possible that one skb->head is backed by a order-3
page.

Is the problem coming from xen_netbk_count_skb_slots() ?

Give me more information if you want me to help.