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

[Frediano Ziglio <freddy77@xxxxxxxxx>]

2014-11-10 16:42 GMT+00:00 David Vrabel <david.vrabel@xxxxxxxxxx>:

On 10/11/14 16:39, Zoltan Kiss wrote:
>
> The BUG_ON suggested by Stefan would be still reasonable:
>
> BUG_ON(((page-compound_head(page))*PAGE_SIZE)+offset+len >
> PAGE_SIZE<<compound_order(compound_head(page)));

Well, it wouldn't trigger but I don't think it is useful any more.

David

Looks like this structure was designed to just contains physical single pages and was extended to support compound pages however there are still some functions that assume the usage of single pages. Just for instance the offset/size of fragments are computed from PAGE_SIZE. On x86 (4KB pages) if your compound page is bigger than 64KB you cannot fully use for a fragment as offset/size are 16 bits. Some functions in skbuff.c use kmap which is limited to physical page.

Frediano

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