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

[Wei Liu <wei.liu2@citrix.com>]

On Fri, May 30, 2014 at 12:28:53PM +0000, David Laight wrote:
> 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?
> 

You're right, this is a valid concern.