Re: [PATCH net-next v8 4/4] skbuff: Optimization of SKB coalescing for page pool

[Liang Chen <liangchen.linux@gmail.com>]

On Tue, Dec 12, 2023 at 4:14 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon, 11 Dec 2023 09:46:55 +0200 Ilias Apalodimas wrote:
> > As I said in the past the patch look correct. I don't like the fact
> > that more pp internals creep into the default network stack, but
> > perhaps this is fine with the bigger adoption?
> > Jakub any thoughts/objections?
>
> Now that you asked... the helper does seem to be in sort of
> a in-between state of being skb specific.
>
> What worries me is that this:
>
> +/**
> + * skb_pp_frag_ref() - Increase fragment reference count of a page
> + * @page:      page of the fragment on which to increase a reference
> + *
> + * Increase fragment reference count (pp_ref_count) on a page, but if it is
> + * not a page pool page, fallback to increase a reference(_refcount) on a
> + * normal page.
> + */
> +static void skb_pp_frag_ref(struct page *page)
> +{
> +       struct page *head_page = compound_head(page);
> +
> +       if (likely(is_pp_page(head_page)))
> +               page_pool_ref_page(head_page);
> +       else
> +               page_ref_inc(head_page);
> +}
>
> doesn't even document that the caller must make sure that the skb
> which owns this page is marked for pp recycling. The caller added
> by this patch does that, but we should indicate somewhere that doing
> skb_pp_frag_ref() for frag in a non-pp-recycling skb is not correct.
>
> We can either lean in the direction of making it less skb specific,
> put the code in page_pool.c / helpers.h and make it clear that the
> caller has to be careful.
> Or we make it more skb specific, take a skb pointer as arg, and also
> look at its recycling marking..
> or just improve the kdoc.

Thank you for the suggestion! I will proceed with improving the kdoc.