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

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Date:   Tue, 12 Feb 2019 10:13:30 -0800
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Tariq Toukan <tariqt@...lanox.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Matthew Wilcox <willy@...radead.org>,
        "brouer@...hat.com" <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        "toke@...hat.com" <toke@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC, PATCH] net: page_pool: Don't use page->private to store dma_addr_t

On Tue, Feb 12, 2019 at 7:16 AM Eric Dumazet <eric.dumazet@...il.com> 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.