Re: [PATCH net-next 1/2] page_pool: unify frag page and non-frag page handling

From: Ilias Apalodimas
Date: Fri May 26 2023 - 11:38:46 EST

On Fri, May 26, 2023 at 08:35:24PM +0800, Yunsheng Lin wrote:
> On 2023/5/26 20:03, Ilias Apalodimas wrote:
> > Hi Yunsheng
> >
> > Apologies for not replying to the RFC, I was pretty busy with internal
> > stuff
> >
> > On Fri, May 26, 2023 at 05:26:14PM +0800, Yunsheng Lin wrote:
> >> Currently page_pool_dev_alloc_pages() can not be called
> >> when PP_FLAG_PAGE_FRAG is set, because it does not use
> >> the frag reference counting.
> >>
> >> As we are already doing a optimization by not updating
> >> page->pp_frag_count in page_pool_defrag_page() for the
> >> last frag user, and non-frag page only have one user,
> >> so we utilize that to unify frag page and non-frag page
> >> handling, so that page_pool_dev_alloc_pages() can also
> >> be called with PP_FLAG_PAGE_FRAG set.
> >
> > What happens here is clear. But why do we need this? Do you have a
> > specific use case in mind where a driver will call
> > page_pool_dev_alloc_pages() and the PP_FLAG_PAGE_FRAG will be set?
> Actually it is about calling page_pool_alloc_pages() in
> page_pool_alloc_frag() in patch 2, the use case is the
> veth using page frag support. see:

Ok I missed that patch.

> > If that's the case isn't it a better idea to unify the functions entirely?
> As about, page_pool_alloc_frag() does seems to be a superset of
> page_pool_alloc_pages() after this patchset as my understanding.
> If the page_pool_alloc_frag() API turns out to be a good API for
> the driver, maybe we can phase out *page_pool_alloc_pages() as
> time goes by?

Looking at patch 2/2 it seems a bit wasteful. At the moment only hns3 uses
fragments and the length of the allocation seems static. But if someone
else chooses to allocate a > 2048 packet why should it allocate a page?

I just think it's a bit confusing since we have a flag on the pool for page
fragments, but then we violate it when it suits us.