linux-kernel - Re: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic in page_pool_page_is

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Message-ID: <b7efa56b-e9fd-4ca6-9ecf-0d5f15b8d0c1@gmail.com>
Date: Wed, 28 May 2025 10:20:29 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Byungchul Park <byungchul@...com>
Cc: Mina Almasry <almasrymina@...gle.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,
 harry.yoo@...cle.com, 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: [PATCH 12/18] page_pool: use netmem APIs to access page->pp_magic
 in page_pool_page_is_pp()

On 5/28/25 10:14, Byungchul Park wrote:
> On Wed, May 28, 2025 at 10:07:52AM +0100, Pavel Begunkov wrote:
>> On 5/28/25 09:14, Byungchul Park wrote:
>>> On Wed, May 28, 2025 at 08:51:47AM +0100, Pavel Begunkov wrote:
>>>> On 5/26/25 03:23, Byungchul Park wrote:
>>>>> On Fri, May 23, 2025 at 10:21:17AM -0700, Mina Almasry wrote:
>>>>>> On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@...com> wrote:
>>>>>>>
>>>>>>> To simplify struct page, the effort to seperate 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 accessing page pool members
>>>>>>> of struct page directly, but use safe APIs for the purpose.
>>>>>>>
>>>>>>> Use netmem_is_pp() instead of directly accessing page->pp_magic in
>>>>>>> page_pool_page_is_pp().
>>>>>>>
>>>>>>> Signed-off-by: Byungchul Park <byungchul@...com>
>>>>>>> ---
>>>>>>>     include/linux/mm.h   | 5 +----
>>>>>>>     net/core/page_pool.c | 5 +++++
>>>>>>>     2 files changed, 6 insertions(+), 4 deletions(-)
>>>>>>>
>>>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>>>> index 8dc012e84033..3f7c80fb73ce 100644
>>>>>>> --- a/include/linux/mm.h
>>>>>>> +++ b/include/linux/mm.h
>>>>>>> @@ -4312,10 +4312,7 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
>>>>>>>     #define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
>>>>>>>
>>>>>>>     #ifdef CONFIG_PAGE_POOL
>>>>>>> -static inline bool page_pool_page_is_pp(struct page *page)
>>>>>>> -{
>>>>>>> -       return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
>>>>>>> -}
>>>>>>
>>>>>> I vote for keeping this function as-is (do not convert it to netmem),
>>>>>> and instead modify it to access page->netmem_desc->pp_magic.
>>>>>
>>>>> Once the page pool fields are removed from struct page, struct page will
>>>>> have neither struct netmem_desc nor the fields..
>>>>>
>>>>> So it's unevitable to cast it to netmem_desc in order to refer to
>>>>> pp_magic.  Again, pp_magic is no longer associated to struct page.
>>>>>
>>>>> Thoughts?
>>>>
>>>> Once the indirection / page shrinking is realized, the page is
>>>> supposed to have a type field, isn't it? And all pp_magic trickery
>>>> will be replaced with something like
>>>>
>>>> page_pool_page_is_pp() { return page->type == PAGE_TYPE_PP; }
>>>
>>> Agree, but we need a temporary solution until then.  I will use the
>>> following way for now:
>>
>> The question is what is the problem that you need another temporary
>> solution? If, for example, we go the placeholder way, page_pool_page_is_pp()
> 
> I prefer using the place-holder, but Matthew does not.  I explained it:
> 
>     https://lore.kernel.org/all/20250528013145.GB2986@system.software.com/
> 
> Now, I'm going with the same way as the other approaches e.g. ptdesc.

Sure, but that doesn't change my point

-- 
Pavel Begunkov