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

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Date:   Mon, 11 Feb 2019 08:53:19 +0000
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Matthew Wilcox <willy@...radead.org>
CC:     David Miller <davem@...emloft.net>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "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 2/7/2019 11:42 PM, Ilias Apalodimas wrote:
> Hi Matthew,
> 
> On Thu, Feb 07, 2019 at 01:34:00PM -0800, Matthew Wilcox wrote:
>> On Thu, Feb 07, 2019 at 01:25:19PM -0800, David Miller wrote:
>>> From: Ilias Apalodimas <ilias.apalodimas@...aro.org>
>>> Date: Thu, 7 Feb 2019 17:20:34 +0200
>>>
>>>> Well updating struct page is the final goal, hence the comment. I am mostly
>>>> looking for opinions here since we are trying to store dma addresses which are
>>>> irrelevant to pages. Having dma_addr_t definitions in mm-related headers is a
>>>> bit controversial isn't it ? If we can add that, then yes the code would look
>>>> better
>>>
>>> I fundamentally disagree.
>>>
>>> One of the core operations performed on a page is mapping it so that a device
>>> and use it.
>>>
>>> Why have ancillary data structure support for this all over the place, rather
>>> than in the common spot which is the page.
>>>
>>> A page really is not just a 'mm' structure, it is a system structure.
>>
>> +1
>>
>> The fundamental point of computing is to do I/O.
> Ok, great that should sort it out then.
> I'll use your proposal and base the patch on that.
> 
> Thanks for taking the time with this
> 
> /Ilias
> 

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).

I would replace this dma field with a pointer to an extensible struct, 
that would contain the dma mapping (and other stuff in the near future).
This pointer fits perfectly with the existing unsigned long private; 
they can share the memory, for both 32- and 64-bits systems.

The only downside is one more pointer de-reference. This should be perf 
tested.
However, when introducing the page refcnt bias optimization into 
page_pool, I believe the perf gain would be guaranteed.

Regards,
Tariq