linux-kernel - Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through struct netmem_desc in page_pool_page_is

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Message-ID: <a8d40a05-db4c-400f-839b-3c6159a1feab@redhat.com>
Date: Mon, 23 Jun 2025 16:58:45 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Byungchul Park <byungchul@...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, almasrymina@...gle.com, 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,
 asml.silence@...il.com, toke@...hat.com, tariqt@...dia.com,
 edumazet@...gle.com, pabeni@...hat.com, saeedm@...dia.com, leon@...nel.org,
 ast@...nel.org, daniel@...earbox.net, 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, hannes@...xchg.org,
 jackmanb@...gle.com
Subject: Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On 23.06.25 13:13, Zi Yan wrote:
> 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@...com>
>>>> Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>
>>>> Reviewed-by: Mina Almasry <almasrymina@...gle.com>
>>>> Reviewed-by: Pavel Begunkov <asml.silence@...il.com>
>>>> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>>>> Acked-by: Harry Yoo <harry.yoo@...cle.com>
>>>> ---
>>>>    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.

My question is rather, whether there is some higher-level freeing path 
for netmem_desc where we could check that instead (IOW, earlier).

Or is it really arbitrary put_page() (IOW, we assume that many possible 
references can be held)?

-- 
Cheers,

David / dhildenb