netdev - Re: [PATCH net-next v11 0/6] introduce page_pool

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Date: Wed, 18 Oct 2023 19:47:16 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <davem@...emloft.net>, <pabeni@...hat.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, Matthias Brugger
	<matthias.bgg@...il.com>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>, <bpf@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-mediatek@...ts.infradead.org>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net-next v11 0/6] introduce page_pool_alloc() related API

On 2023/10/17 23:13, Jakub Kicinski wrote:
> On Tue, 17 Oct 2023 15:56:48 +0800 Yunsheng Lin wrote:
>>> And I can't figure out now what the "cache" in the name is referring to.
>>> Looks like these are just convenience wrappers which return VA instead
>>> of struct page..  
>>
>> Yes, it is corresponding to some API like napi_alloc_frag() returning va
>> instead of 'struct page' mentioned in patch 5.
>>
>> Anyway, naming is hard, any suggestion for a better naming is always
>> welcomed:)
> 
> I'd just throw a _va (for virtual address) at the end. And not really

_va seems fine:)

> mention it in the documentation. Plus the kdoc of the function should
> say that this is just a thin wrapper around other page pool APIs, and
> it's safe to mix it with other page pool APIs?

I am not sure I understand what do 'safe' and 'mix' mean here.

For 'safe' part, I suppose you mean if there is a va accociated with
a 'struct page' without calling some API like kmap()? For that, I suppose
it is safe when the driver is calling page_pool API without the
__GFP_HIGHMEM flag. Maybe we should mention that in the kdoc and give a
warning if page_pool_*alloc_va() is called with the __GFP_HIGHMEM flag?

For the 'mix', I suppose you mean the below:
1. Allocate a page with the page_pool_*alloc_va() API and free a page with
   page_pool_free() API.
2. Allocate a page with the page_pool_*alloc() API and free a page with
   page_pool_free_va() API.

For 1, it seems it is ok as some virt_to_head_page() and page_address() call
between va and 'struct page' does not seem to change anything if we have
enforce page_pool_*alloc_va() to be called without the __GFP_HIGHMEM flag.

For 2, If the va is returned from page_address() which the allocation API is
called without __GFP_HIGHMEM flag. If not, the va is from kmap*()? which means
we may be calling page_pool_free_va() before kunmap*()? Is that possible?

> .
>