linux-kernel - Re: [RFC v4 18/18] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <9138df77-fe7d-430b-a369-05453a76bdb4@gmail.com>
Date: Thu, 5 Jun 2025 21:16:12 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Mina Almasry <almasrymina@...gle.com>, Harry Yoo <harry.yoo@...cle.com>
Cc: Byungchul Park <byungchul@...com>, willy@...radead.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 kernel_team@...ynix.com, kuba@...nel.org, ilias.apalodimas@...aro.org,
 hawk@...nel.org, akpm@...ux-foundation.org, davem@...emloft.net,
 john.fastabend@...il.com, andrew+netdev@...n.ch, toke@...hat.com,
 tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com,
 saeedm@...dia.com, leon@...nel.org, ast@...nel.org, daniel@...earbox.net,
 david@...hat.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 vbabka@...e.cz, rppt@...nel.org, surenb@...gle.com, mhocko@...e.com,
 horms@...nel.org, linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
 vishal.moola@...il.com
Subject: Re: [RFC v4 18/18] page_pool: access ->pp_magic through struct
 netmem_desc in page_pool_page_is_pp()

On 6/5/25 20:47, Mina Almasry wrote:
> On Thu, Jun 5, 2025 at 4:49 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>>
>> On Thu, Jun 05, 2025 at 11:56:14AM +0100, Pavel Begunkov wrote:
>>> On 6/4/25 03:52, Byungchul Park wrote:
>>>> To simplify struct page, the effort to separate its own descriptor from
>>>> struct page is required and the work for page pool is on going.
>>>>
>>>> To achieve that, all the code should avoid directly accessing page pool
>>>> members of struct page.
>>>
>>> Just to clarify, are we leaving the corresponding struct page fields
>>> for now until the final memdesc conversion is done?
>>
>> Yes, that's correct.
>>
>>> If so, it might be better to leave the access in page_pool_page_is_pp()
>>> to be "page->pp_magic", so that once removed the build fails until
>>> the helper is fixed up to use the page->type or so.
>>
>> When we truly separate netmem from struct page, we won't have 'lru' field
>> in memdesc (because not all types of memory are on LRU list),
>> so NETMEM_DESC_ASSERT_OFFSET(lru, pp_magic) should fail.
>>
>> And then page_pool_page_is_pp() should be changed to check lower bits
>> of memdesc pointer to identify its type.
>>
> 
> Oh boy, I'm not sure that works. We already do LSB tricks with
> netmem_ref to tell what kind of ref it is. I think the LSB pointer
> tricks with netmem_ref and netmem_desc may trample each other's toes.
> I guess we'll cross that bridge when we get to it...

I believe Harry wants to tag struct page::memdesc, while
netmem is tagging the struct page pointer / net_iov.

-- 
Pavel Begunkov