Subject: Re: [RFC, PATCH] net: page_pool: Don't use page->private to store dma_addr_t

From: Ilias Apalodimas <[email-masked]>
Date: Tue, 12 Feb 2019 20:20:31 +0200

Hi Alexander, 

On Tue, Feb 12, 2019 at 10:13:30AM -0800, Alexander Duyck wrote:
> On Tue, Feb 12, 2019 at 7:16 AM Eric Dumazet <[email-masked]> wrote:
> >
> >
> >
> > On 02/12/2019 04:39 AM, Tariq Toukan wrote:
> > >
> > >
> > > On 2/11/2019 7:14 PM, Eric Dumazet wrote:
> > >>
> > >>
> > >> On 02/11/2019 12:53 AM, Tariq Toukan wrote:
> > >>>
> > >>
> > >>> Hi,
> > >>>
> > >>> It's great to use the struct page to store its dma mapping, but I am
> > >>> worried about extensibility.
> > >>> page_pool is evolving, and it would need several more per-page fields.
> > >>> One of them would be pageref_bias, a planned optimization to reduce the
> > >>> number of the costly atomic pageref operations (and replace existing
> > >>> code in several drivers).
> > >>>
> > >>
> > >> But the point about pageref_bias is to place it in a different cache line than "struct page"
> > >>
> > >> The major cost is having a cache line bouncing between producer and consumer.
> > >>
> > >
> > > pageref_bias is meant to be dirtied only by the page requester, i.e. the
> > > NIC driver / page_pool.
> > > All other components (basically, SKB release flow / put_page) should
> > > continue working with the atomic page_refcnt, and not dirty the
> > > pageref_bias.
> >
> > This is exactly my point.
> >
> > You suggested to put pageref_bias in struct page, which breaks this completely.
> >
> > pageref_bias is better kept in a driver structure, with appropriate prefetching
> > since most NIC use a ring buffer for their queues.
> >
> > The dma address _can_ be put in the struct page, since the driver does not dirty it
> > and does not even read it when page can be recycled.
> 
> Instead of maintaining the pageref_bias in the page itself it could be
> maintained in some sort of separate structure. You could just maintain
> a pointer to a slot in an array somewhere. Then you can still access
> it if needed, the pointer would be static for as long as it is in the
> page pool, and you could invalidate the pointer prior to removing the
> bias from the page.

I think that's what Tariq was suggesting in the first place.

/Ilias