Re: [Xen-devel] BUG in xennet_make_frags with paged skb data

From: Zoltan Kiss
Date: Fri Nov 07 2014 - 04:25:57 EST


Hi,

AFAIK in this scenario your skb frag is wrong. The page pointer should point to the original compound page (not a member of it), and offset should be set accordingly.
For example, if your compound page is 16K (4 page), then the page pointer should point to the first page, and if the data starts at the 3rd page, then offset should be >8K

Zoli

On 06/11/14 21:49, Seth Forshee wrote:
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

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