Re: 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:

quoted

On Tue, 2012-10-09 at 14:47 +0100, Ian Campbell wrote:

quoted

Hi Eric,

Hi Ian

quoted

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
either.

Ian.