BUG in xennet_make_frags with paged skb data

[Seth Forshee <seth.forshee@canonical.com>]

We've had several reports of hitting the following BUG_ON in
xennet_make_frags with 3.2 and 3.13 kernels (I'm currently awaiting
results of testing with 3.17):

        /* Grant backend access to each skb fragment page. */
        for (i = 0; i < frags; i++) {
                skb_frag_t *frag = skb_shinfo(skb)->frags + i;
                struct page *page = skb_frag_page(frag);

                len = skb_frag_size(frag);
                offset = frag->page_offset;

                /* Data must not cross a page boundary. */
                BUG_ON(len + offset > PAGE_SIZE<<compound_order(page));

When this happens the page in question is a "middle" page in a compound
page (i.e. it's a tail page but not the last tail page), and the data is
fully contained within the compound page. The data does however cross
the hardware page boundary, and since compound_order evaluates to 0 for
tail pages the check fails.

In going over this I've been unable to determine whether the BUG_ON in
xennet_make_frags is incorrect or the paged skb data is wrong. I can't
find that it's documented anywhere, and the networking code itself is a
bit ambiguous when it comes to compound pages. On the one hand
__skb_fill_page_desc specifically handles adding tail pages as paged
data, but on the other hand skb_copy_bits kmaps frag->page.p which could
fail with data that extends into another page.

Can anyone explain what the rules are here? My best guess based on
skb_copy_bits is that paged data should never cross the hardware page
boundary, but I'm not really sure how all of this works out when dealing
with compound pages.

Thanks,
Seth