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 07:15:55 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tariq Toukan <tariqt@...lanox.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Matthew Wilcox <willy@...radead.org>,
        "brouer@...hat.com" <brouer@...hat.com>
Cc:     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 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.