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

[Harry Yoo]

On Wed, Jun 11, 2025 at 03:30:28PM +0100, Pavel Begunkov wrote:
> On 6/10/25 02:45, Byungchul Park wrote:
> > On Mon, Jun 09, 2025 at 10:39:06AM -0700, Mina Almasry wrote:
> > > On Sun, Jun 8, 2025 at 9:32 PM Byungchul Park <byungchul@xxxxxx> 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>
> > > > ---
> > > >   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 e51dba8398f7..f23560853447 100644
> > > > --- a/include/linux/mm.h
> > > > +++ b/include/linux/mm.h
> > > > @@ -4311,16 +4311,4 @@ 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 d84ab624b489..8f354ae7d5c3 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
> > > > +
> > > >   /* net_iov */
> > > > 
> > > >   DECLARE_STATIC_KEY_FALSE(page_pool_mem_providers);
> > > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > > index 4f29e393f6af..be0752c0ac92 100644
> > > > --- a/mm/page_alloc.c
> > > > +++ b/mm/page_alloc.c
> > > > @@ -55,6 +55,7 @@
> > > >   #include <linux/delayacct.h>
> > > >   #include <linux/cacheinfo.h>
> > > >   #include <linux/pgalloc_tag.h>
> > > > +#include <net/netmem.h>
> > > 
> > > mm files starting to include netmem.h is a bit interesting. I did not
> > > expect/want dependencies outside of net. If anything the netmem stuff
> > > include linux/mm.h
> > 
> > That's what I also concerned.  However, now that there are no way to
> > check the type of memory in a general way but require to use one of pp
> > fields, page_pool_page_is_pp() should be served by pp code e.i. network
> > subsystem.
> > 
> > This should be changed once either 1) mm provides a general way to check
> > the type or 2) pp code is moved to mm code.  I think this approach
> > should acceptable until then.
> 
> I'd argue in the end the helper should be in mm.h as mm is going to
> dictate how to check the type and keep them enumerated.
>
> Reviewed-by: Pavel Begunkov <asml.silence@xxxxxxxxx>

Acked-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

-- 
Cheers,
Harry / Hyeonggon