Message-ID: <20250623112530.GA67291@system.software.com>
Date: Mon, 23 Jun 2025 20:25:30 +0900
From: Byungchul Park <byungchul@...com>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, willy@...radead.org,
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Subject: Re: [PATCH net-next v6 9/9] page_pool: access ->pp_magic through
 struct netmem_desc in page_pool_page_is_pp()

On Mon, Jun 23, 2025 at 07:13:21AM -0400, 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.

Indeed..  So I think it'd be better to leave the check as is until we
will be fully convinced on that issue, I ideally agree with David's
opinion tho.

	Byungchul

> [1] https://lore.kernel.org/linux-mm/4d35bda2-d032-49db-bb6e-b1d70f10d436@kernel.org/
> 
> --
> Best Regards,
> Yan, Zi

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