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

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Message-ID: <b8efc2ce-8856-2c9b-2a8c-edf2a819ebe5@huawei.com>
Date:   Fri, 18 Aug 2023 16:46:01 +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 19:43, Ilias Apalodimas wrote:
>>>>>>
>>>>>> 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?
> 
> It's been a while since I looked at this, but doesn't __xdp_return()
> still sync the entire page if the mem type comes from page_pool?

Yes, I checked that too.
It is supposed to sync the entire page if the mem type comes from page_pool,
as it depend on the last freed frag to do the sync_for_device operation.