netdev - Re: [PATCH net-next v6 1/6] page_pool: frag API support for 32-bit arch with 64-bit DMA

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Date: Thu, 17 Aug 2023 17:05:38 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Lorenzo Bianconi
	<lorenzo@...nel.org>, Alexander Duyck <alexander.duyck@...il.com>, Liang Chen
	<liangchen.linux@...il.com>, Alexander Lobakin
	<aleksander.lobakin@...el.com>, Saeed Mahameed <saeedm@...dia.com>, Leon
 Romanovsky <leon@...nel.org>, Eric Dumazet <edumazet@...gle.com>, Jesper
 Dangaard Brouer <hawk@...nel.org>, <linux-rdma@...r.kernel.org>
Subject: Re: [PATCH net-next v6 1/6] page_pool: frag API support for 32-bit
 arch with 64-bit DMA

On 2023/8/17 1:01, Ilias Apalodimas wrote:
> On Wed, 16 Aug 2023 at 15:49, Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>
>> On 2023/8/16 19:26, Ilias Apalodimas wrote:
>>> Hi Yunsheng
>>>
>>> On Mon, 14 Aug 2023 at 15:59, Yunsheng Lin <linyunsheng@...wei.com> wrote:
>>>>
>>>> Currently page_pool_alloc_frag() is not supported in 32-bit
>>>> arch with 64-bit DMA because of the overlap issue between
>>>> pp_frag_count and dma_addr_upper in 'struct page' for those
>>>> arches, which seems to be quite common, see [1], which means
>>>> driver may need to handle it when using frag API.
>>>
>>> That wasn't so common. IIRC it was a single TI platform that was breaking?
>>
>> I am not so sure about that as grepping 'ARM_LPAE' has a long
>> list for that.
> 
> Shouldn't we be grepping for CONFIG_ARCH_DMA_ADDR_T_64BIT and
> PHYS_ADDR_T_64BIT to find the affected platforms?  Why LPAE?


I used the key in the  original report:

https://www.spinics.net/lists/netdev/msg779890.html

>> Please see the bisection report below about a boot failure on
>> rk3288-rock2-square which is pointing to this patch.  The issue
>> appears to only happen with CONFIG_ARM_LPAE=y.

grepping the 'CONFIG_PHYS_ADDR_T_64BIT' seems to be more common?
https://elixir.free-electrons.com/linux/v6.4-rc6/K/ident/CONFIG_PHYS_ADDR_T_64BIT

> 
>>
>>>
>>>>
>>>> In order to simplify the driver's work when using frag API
>>>> this patch allows page_pool_alloc_frag() to call
>>>> page_pool_alloc_pages() to return pages for those arches.
>>>
>>> Do we have any use cases of people needing this?  Those architectures
>>> should be long dead and although we have to support them in the
>>> kernel,  I don't personally see the advantage of adjusting the API to
>>> do that.  Right now we have a very clear separation between allocating
>>> pages or fragments.   Why should we hide a page allocation under a
>>> frag allocation?  A driver writer can simply allocate pages for those
>>> boards.  Am I the only one not seeing a clean win here?
>>
>> It is also a part of removing the per page_pool PP_FLAG_PAGE_FRAG flag
>> in this patchset.
> 
> Yes, that happens *because* of this patchset.  I am not against the
> change.  In fact, I'll have a closer look tomorrow.  I am just trying
> to figure out if we really need it.  When the recycling patches were
> introduced into page pool we had a very specific reason.  Due to the
> XDP verifier we *had* to allocate a packet per page.  That was

Did you mean a xdp frame containing a frag page can not be passed to the
xdp core?
What is exact reason why the XDP verifier need a packet per page?
Is there a code block that you can point me to?

I wonder if it is still the case for now, as bnxt and mlx5 seems to be
supporting frag page and xdp now.

> expensive so we added the recycling capabilities to compensate and get
> some performance back. Eventually we added page fragments and had a
> very clear separation on the API.
> 
> Regards
> /Ilias
>>
>>>
>>> Thanks
>>> /Ilias
>>>
> .
>