Messages in this thread | | | Date | Thu, 6 Nov 2014 15:49:40 -0600 | From | Seth Forshee <> | Subject | BUG in xennet_make_frags with paged skb data |
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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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