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

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Date:   Wed, 13 Feb 2019 08:50:54 +0000
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Alexander Duyck <alexander.duyck@...il.com>
CC:     Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        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 2/12/2019 8:20 PM, Ilias Apalodimas wrote:
> 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 <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.
> 
> I think that's what Tariq was suggesting in the first place.
> 
> /Ilias
> 

Correct.
But not relevant anymore, as it won't work for other reasons.

Tariq