linux-kernel - Re: [PATCH net-next 1/2] page_pool: unify frag page and non-frag page handling

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Message-ID: <5640bab0-d2f9-50ee-f3e2-eb0f86b144dc@huawei.com>
Date:   Fri, 26 May 2023 20:35:24 +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>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/2] page_pool: unify frag page and non-frag page
 handling

On 2023/5/26 20:03, Ilias Apalodimas wrote:
> Hi Yunsheng
> 
> Apologies for not replying to the RFC,  I was pretty busy with internal
> stuff
> 
> On Fri, May 26, 2023 at 05:26:14PM +0800, Yunsheng Lin wrote:
>> Currently page_pool_dev_alloc_pages() can not be called
>> when PP_FLAG_PAGE_FRAG is set, because it does not use
>> the frag reference counting.
>>
>> As we are already doing a optimization by not updating
>> page->pp_frag_count in page_pool_defrag_page() for the
>> last frag user, and non-frag page only have one user,
>> so we utilize that to unify frag page and non-frag page
>> handling, so that page_pool_dev_alloc_pages() can also
>> be called with PP_FLAG_PAGE_FRAG set.
> 
> What happens here is clear.  But why do we need this?  Do you have a
> specific use case in mind where a driver will call
> page_pool_dev_alloc_pages() and the PP_FLAG_PAGE_FRAG will be set?

Actually it is about calling page_pool_alloc_pages() in
page_pool_alloc_frag() in patch 2, the use case is the
veth using page frag support. see:

https://patchwork.kernel.org/project/netdevbpf/patch/d3ae6bd3537fbce379382ac6a42f67e22f27ece2.1683896626.git.lorenzo@kernel.org/

> If that's the case isn't it a better idea to unify the functions entirely?

As about, page_pool_alloc_frag() does seems to be a superset of
page_pool_alloc_pages() after this patchset as my understanding.
If the page_pool_alloc_frag() API turns out to be a good API for
the driver, maybe we can phase out *page_pool_alloc_pages() as
time goes by?