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Re: [Xen-devel] compound skb frag pages appearing in start_xmit

On Tue, 2012-10-09 at 15:01 +0100, Eric Dumazet wrote:
> 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.

The interesting code is in netbk_gop_skb(), specifically the two calls
to netbk_gop_frag_copy.

netbk_gop_frag_copy can only copy order-0 pages to the peer since they
go over a shared ring transport which can only deal in order-0 pages.

For the SKB head there is a loop which handles order>0 heads, I suspect
we just need something similar for the frag case.

Although see my question in the other response about the maximum number
of frags we can have when order is > 0 since if using larger pages
causes us to end up with a much larger number of order-0 pages once
we've broken them up then we have a problem and I need to put my
thinking cap on a bit (perhaps substantially) tighter.

Konrad, it looks like netfront has a similar issue in
xennet_make_frags() since it doesn't shatter large order mappings


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