Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and big packets in virtio_net

Shirley Ma
2009-11-23T16:18:10Z

On Mon, 2009-11-23 at 11:43 +0200, Michael S. Tsirkin wrote:

> should be !npage->private
> and nesting is too deep here:
> this is cleaner in a give_a_page subroutine
> as it was.

This will be addressed with Rusty's comment.

> > + /* use private to chain big packets */
>
> packets? or pages?

Will change it to chain pages for big packets

> > + p->private = (unsigned long)0;
>
> the comment is not really helpful:
> you say you use private to chain but 0 does not
> chain anything. You also do not need the cast to long?

Ok.

> > + if (len > (PAGE_SIZE - f->page_offset))
>
> brackets around math are not needed.

OK.

> typo
>
> > + * header and data */
Got it.

> please think of a way to get rid of magic constants like 6 and 2
> here and elsewhere.

Will do.

> replace MAX_SKB_FRAGS + 2 with something symbolic? We have it in 2 palces now.
> And comment.

Ok, I can change it.

> terrible goto based loop
> move stuff into subfunction, it will be much
> more manageable, and convert this to a simple
> for loop.

Will change it to different functions based on Rusty's comment.

> and here it is MAX_SKB_FRAGS + 1

I think I should use MAX_SKB_FRAGS + 2 instead. Now I only use
MAX_SKB_FRAGS + 1 but allocated + 2.

> > + page->private = (unsigned long)first_page;
> > + first_page = page;
> > + if (--i == 1) {
>
> this is pretty hairy ... has to be this way?
> What you are trying to do here
> is fill buffer with pages, in a loop, with first one
> using a partial page, and then add it.
> Is that it?
Yes.

> So please code this in a straight forward manner.
> it should be as simple as:

> offset = XXX
> for (i = 0; i < MAX_SKB_FRAGS + 2; ++i) {
>
> sg_set_buf(sg + i, p + offset, PAGE_SIZE - offset);
> offset = 0;
>
> }
Ok, looks more neat.

> space around +
> sg + 1 here is same as &sg[i] in fact?

Ok.

> callback -> destructor?

Ok.

I will integrate these comments with Rusty's and resubmit the patch set.

Thanks
Shirley