Re: [Xen-devel] [PATCH net-next] xen-netfront: try linearizing SKB if it occupies too many slots

[David Laight <David.Laight@xxxxxxxxxx>]

From: Wei Liu
> On Fri, May 30, 2014 at 10:06:48AM +0200, Stefan Bader wrote:
> [...]
> > I had been idly wondering about this onwards. And trying to understand the 
> > whole
> > skb handling environment, I tried to come up with some idea as well. It may 
> > be
> > totally stupid and using the wrong assumptions. It seems to work in the 
> > sense
> > that things did not blow up into my face immediately and somehow I did not 
> > see
> > dropped packages due to the number of slots either.
> > But again, I am not sure I am doing the right thing. The idea was to just 
> > try to
> > get rid of so many compound pages (which I believe are the only ones that 
> > can
> > have an offset big enough to allow some alignment savings)...
> >
> > -Stefan
> >
> 
> Thanks. I think the general idea is OK, but it still involves
> unnecessary page allocation. We don't actually need to get rid of
> compound page by replacing it with a new page, we just need to make sure
> the data inside is aligned.
> 
> If you look at xennet_make_frags, it only grants the 4K page which
> contains data. I presume a simple memove would be better than alloc_page
> + memcpy. What do you think?
> 
> Like:
>    memmove(page_address(fpage), page_address(fpage)+offset, size);
>    frag->page_offset = 0;

Isn't the rest of the page likely to contain fragments of other ethernet
frames?  Even possibly of other data?

        David




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