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

[Mina Almasry]

On Mon, Jun 23, 2025 at 10:05 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
>
> On 6/23/25 15:58, David Hildenbrand wrote:
> > 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@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.
>
> As you said, it's just a sanity check, all page pool pages should
> be freed by the networking code. It checks the ownership with
> netmem_is_pp(), which is basically the same as page_pool_page_is_pp()
> but done though some aliasing.
>
> static inline bool netmem_is_pp(netmem_ref netmem)
> {
>         return (netmem_get_pp_magic(netmem) & PP_MAGIC_MASK) == PP_SIGNATURE;
> }
>
> I assume there is no point in moving the check to skbuff.c as it
> already does exactly same test, but we can probably just kill it.
>

Even if we do kill it, maybe lets do that in a separate patch, and
maybe a separate series. I would recommend not complicating this one?

Also, AFAIU, this is about removing/moving the checks in
bad_page_reason() and page_expected_state()? I think this check does
fire sometimes. I saw at least 1 report in the last year of a
bad_page_reason() check firing because the page_pool got its
accounting wrong and released a page to the buddy allocator early, so
maybe that new patch that removes that check should explain why this
check is no longer necessary.

-- 
Thanks,
Mina