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

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Date:   Fri, 26 May 2023 18:38:22 +0300
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/2] page_pool: unify frag page and non-frag
 page handling

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:
>
> https://patchwork.kernel.org/project/netdevbpf/patch/d3ae6bd3537fbce379382ac6a42f67e22f27ece2.1683896626.git.lorenzo@kernel.org/

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.

Thanks
/Ilias