Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is_pp()

[Zi Yan]

On 23 Jun 2025, at 6:16, Byungchul Park wrote:

> On Mon, Jun 23, 2025 at 11:16:43AM +0200, David Hildenbrand wrote:
>> On 20.06.25 06:12, 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.
>>>
>>> Access ->pp_magic through struct netmem_desc instead of directly
>>> accessing it through struct page in page_pool_page_is_pp().  Plus, move
>>> page_pool_page_is_pp() from mm.h to netmem.h to use struct netmem_desc
>>> without header dependency issue.
>>>
>>> Signed-off-by: Byungchul Park <byungchul@xxxxxx>
>>> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
>>> Reviewed-by: Mina Almasry <almasrymina@xxxxxxxxxx>
>>> Reviewed-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
>>> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
>>> Acked-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
>>> ---
>>>   include/linux/mm.h   | 12 ------------
>>>   include/net/netmem.h | 14 ++++++++++++++
>>>   mm/page_alloc.c      |  1 +
>>>   3 files changed, 15 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 0ef2ba0c667a..0b7f7f998085 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -4172,16 +4172,4 @@ 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;
>>> -}
>>> -#else
>>> -static inline bool page_pool_page_is_pp(struct page *page)
>>> -{
>>> -     return false;
>>> -}
>>> -#endif
>>> -
>>>   #endif /* _LINUX_MM_H */
>>> diff --git a/include/net/netmem.h b/include/net/netmem.h
>>> index d49ed49d250b..3d1b1dfc9ba5 100644
>>> --- a/include/net/netmem.h
>>> +++ b/include/net/netmem.h
>>> @@ -56,6 +56,20 @@ NETMEM_DESC_ASSERT_OFFSET(pp_ref_count, pp_ref_count);
>>>    */
>>>   static_assert(sizeof(struct netmem_desc) <= offsetof(struct page, _refcount));
>>>
>>> +#ifdef CONFIG_PAGE_POOL
>>> +static inline bool page_pool_page_is_pp(struct page *page)
>>> +{
>>> +     struct netmem_desc *desc = (struct netmem_desc *)page;
>>> +
>>> +     return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
>>> +}
>>> +#else
>>> +static inline bool page_pool_page_is_pp(struct page *page)
>>> +{
>>> +     return false;
>>> +}
>>> +#endif
>>
>> I wonder how helpful this cleanup is long-term.
>>
>> page_pool_page_is_pp() is only called from mm/page_alloc.c, right?
>
> Yes.
>
>> There, we want to make sure that no pagepool page is ever returned to
>> the buddy.
>>
>> How reasonable is this sanity check to have long-term? Wouldn't we be
>> able to check that on some higher-level freeing path?
>>
>> The reason I am commenting is that once we decouple "struct page" from
>> "struct netmem_desc", we'd have to lookup here the corresponding "struct
>> netmem_desc".
>>
>> ... but at that point here (when we free the actual pages), the "struct
>> netmem_desc" would likely already have been freed separately (remember:
>> it will be dynamically allocated).
>>
>> With that in mind:
>>
>> 1) Is there a higher level "struct netmem_desc" freeing path where we
>> could check that instead, so we don't have to cast from pages to
>> netmem_desc at all.
>
> I also thought it's too paranoiac.  However, I thought it's other issue
> than this work.  That's why I left the API as is for now, it can be gone
> once we get convinced the check is unnecessary in deep buddy.  Wrong?
>
>> 2) How valuable are these sanity checks deep in the buddy?
>
> That was also what I felt weird on.

It seems very useful when I asked last time[1]:

|> We have actually used this at Cloudflare to catch some page_pool bugs.

[1] https://lore.kernel.org/linux-mm/4d35bda2-d032-49db-bb6e-b1d70f10d436@xxxxxxxxxx/

--
Best Regards,
Yan, Zi