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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, May 30, 2014 at 02:28:17PM +0200, Stefan Bader wrote:
> On 30.05.2014 14:11, Wei Liu wrote:
> > 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;
> 
> I was hesitating to do that as I could not really tell whether I can make any
> assumptions about those memory areas. So my cautious guess was to leave the
> original pages alone altogether (maybe whoever owns those has something
> important in the starting area). If I did it right, my new page potentially
> could be a smaller allocation unit than the original, since I only ask for
> something big enough to hold the frag size (ignoring a potential offset over
> full 4K areas).
> 

Thanks for the explanation.

Wei.

> -Stefan

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