Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is_pp()

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/12/25 15:52, David Hildenbrand wrote:
> On 12.07.25 15:58, Pavel Begunkov wrote:
>> On 7/11/25 02:14, Byungchul Park wrote:
>> ...>>> +#ifdef CONFIG_PAGE_POOL
>>>>> +/* XXX: This would better be moved to mm, once mm gets its way to
>>>>> + * identify the type of page for page pool.
>>>>> + */
>>>>> +static inline bool page_pool_page_is_pp(struct page *page)
>>>>> +{
>>>>> +       struct netmem_desc *desc = page_to_nmdesc(page);
>>>>> +
>>>>> +       return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
>>>>> +}
>>>>
>>>> pages can be pp pages (where they have pp fields inside of them) or
>>>> non-pp pages (where they don't have pp fields inside them, because
>>>> they were never allocated from the page_pool).
>>>>
>>>> Casting a page to a netmem_desc, and then checking if the page was a
>>>> pp page doesn't makes sense to me on a fundamental level. The
>>>> netmem_desc is only valid if the page was a pp page in the first
>>>> place. Maybe page_to_nmdesc should reject the cast if the page is not
>>>> a pp page or something.
>>>
>>> Right, as you already know, the current mainline code already has the
>>> same problem but we've been using the werid way so far, in other words,
>>> mm code is checking if it's a pp page or not by using ->pp_magic, but
>>> it's ->lur, ->buddy_list, or ->pcp_list if it's not a pp page.
>>>
>>> Both the mainline code and this patch can make sense *only if* it's
>>> actually a pp page.  It's unevitable until mm provides a way to identify
>>> the type of page for page pool.  Thoughts?
>> Question to mm folks, can we add a new PGTY for page pool and use
>> that to filter page pool originated pages? Like in the incomplete
>> and untested diff below?
> 
> https://lore.kernel.org/all/77c6a6dd-0e03-4b81-a9c7-eaecaa4ebc0b@redhat.com/

Great, then it'll be the right thing to do here. I somehow missed
the post, will add your suggested-by.

-- 
Pavel Begunkov