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

[Zoltan Kiss]

On 07/11/14 12:15, Stefan Bader wrote:
> On 07.11.2014 12:22, Eric Dumazet wrote:
> > On Fri, 2014-11-07 at 09:25 +0000, Zoltan Kiss wrote:
> >
> > Please do not top post.
> >
> > > 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
> >
> > This is not accurate.
> >
> > This BUG_ON() is wrong.
> >
> > It should instead be :
> >
> > BUG_ON(len + offset > PAGE_SIZE<<compound_order(compound_head(page)));
>
> would that not have to be
>
> BUG_ON((page-compound_head(page)*PAGE_SIZE)+offset+len >
> PAGE_SIZE<<compound_order(compound_head(page)));

There should be a parentheses around "page-compound_head(page)".
> since offset is adjusted to start from the tail page in that case.
> > splice() code can generate such cases.
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