Re: [PATCH net-next v4 2/4] page_pool: Allow drivers to hint on SKB recycling

[Eric Dumazet <edumazet@google.com>]

On Wed, May 12, 2021 at 11:50 AM Ilias Apalodimas
<ilias.apalodimas@linaro.org> wrote:
>
> [...]
> > > Since we added an extra argument on __skb_frag_unref() to handle
> > > recycling, update the current users of the function with that.
> >
> > This part could be done with a preliminary patch, only adding this
> > extra boolean, this would keep the 'complex' patch smaller.
>
> Sure
>
> [...]
> > >  #include <linux/uaccess.h>
> > >  #include <trace/events/skb.h>
> > > @@ -645,6 +648,11 @@ static void skb_free_head(struct sk_buff *skb)
> > >  {
> > >     unsigned char *head = skb->head;
> > >
> > > +#if IS_BUILTIN(CONFIG_PAGE_POOL)
> >
> > Why IS_BUILTIN() ?
>
> No reason, we'll replace it with an ifdef
>
> >
> > PAGE_POOL is either y or n
> >
> > IS_ENABLED() would look better, since we use IS_BUILTIN() for the cases where a module might be used.
> >
> > Or simply #ifdef CONFIG_PAGE_POOL
> >
> > > +   if (skb->pp_recycle && page_pool_return_skb_page(head))
> >
> > This probably should be attempted only in the (skb->head_frag) case ?
>
> I think the extra check makes sense.

What do you mean here ?

>
> >
> > Also this patch misses pskb_expand_head()
>
> I am not sure I am following. Misses what? pskb_expand_head() will either
> call skb_release_data() or skb_free_head(), which would either recycle or
> unmap the buffer for us (depending on the page refcnt)

 pskb_expand_head() allocates a new skb->head, from slab.

We should clear skb->pp_recycle for consistency of the skb->head_frag
clearing we perform there.

But then, I now realize you use skb->pp_recycle bit for both skb->head
and fragments,
and rely on this PP_SIGNATURE thing (I note that patch 1 changelog
does not describe why a random page will _not_ have this signature by
bad luck)

Please document/describe which struct page fields are aliased with
page->signature ?

Thanks !