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: <a4ff25cb-e31f-4ed7-a3b9-867b861b17bd@gmail.com>
Date: Wed, 28 May 2025 08:51:47 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Byungchul Park <byungchul@...com>, Mina Almasry <almasrymina@...gle.com>
Cc: 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 5/26/25 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; }


-- 
Pavel Begunkov