Message-ID: <11568ebd-4c00-463e-a2ca-5c75dbd625ef@gmail.com>
Date: Wed, 28 May 2025 11:54:32 +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,
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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 11:44, Byungchul Park wrote:
> On Wed, May 28, 2025 at 10:51:29AM +0100, Pavel Begunkov wrote:
>> On 5/28/25 10:33, Byungchul Park wrote:
>>> On Wed, May 28, 2025 at 10:20:29AM +0100, Pavel Begunkov wrote:
>>>> 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
>>>
>>> What's your point?  The other appoaches do not use place-holders.  I
>>> don't get your point.
>>>
>>> As I told you, I will introduce a new struct, netmem_desc, instead of
>>> struct_group_tagged() on struct net_iov, and modify the static assert on
>>> the offsets to keep the important fields between struct page and
>>> netmem_desc.
>>>
>>> Then, is that following your point?  Or could you explain your point in
>>> more detail?  Did you say other points than these?
>>
>> Then please read the message again first. I was replying to th
>> aliasing with "lru", and even at the place you cut the message it
>> says "for example", which was followed by "You should be able to
>> do the same with the overlay option.".
> 
> With struct_group_tagged() on struct net_iov, no idea about how to.
> However, it's doable with a new separate struct, struct netmem_desc.

static inline bool page_pool_page_is_pp(struct page *page)
{
	pp_magic = page_to_netdesc(page)->pp_magic;
	return pp_magic == ...;
}

page_to_netdesc() is either casting directly in case of full page
overlays, or "&page->netdesc" for the placeholder option.

-- 
Pavel Begunkov

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