Re: [PATCH v5] mm: introduce a new page type for page pool in page type

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Jesper,

On Tue, 17 Mar 2026 at 11:20, Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>
>
>
>
> On 16/03/2026 23.31, Byungchul Park wrote:
> > Currently, the condition 'page->pp_magic == PP_SIGNATURE' is used to
> > determine if a page belongs to a page pool.  However, with the planned
> > removal of @pp_magic, we should instead leverage the page_type in struct
> > page, such as PGTY_netpp, for this purpose.
> >
> > Introduce and use the page type APIs e.g. PageNetpp(), __SetPageNetpp(),
> > and __ClearPageNetpp() instead, and remove the existing APIs accessing
> > @pp_magic e.g. page_pool_page_is_pp(), netmem_or_pp_magic(), and
> > netmem_clear_pp_magic().
> >
> > Plus, add @page_type to struct net_iov at the same offset as struct page
> > so as to use the page_type APIs for struct net_iov as well.  While at it,
> > reorder @type and @owner in struct net_iov to avoid a hole and
> > increasing the struct size.
> >
> > This work was inspired by the following link:
> >
> >    https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@gmail.com/
> >
> > While at it, move the sanity check for page pool to on the free path.
> >
> [...]
> see below
>
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 9f2fe46ff69a1..ee81f5c67c18f 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1044,7 +1044,6 @@ static inline bool page_expected_state(struct page *page,
> >   #ifdef CONFIG_MEMCG
> >                       page->memcg_data |
> >   #endif
> > -                     page_pool_page_is_pp(page) |
> >                       (page->flags.f & check_flags)))
> >               return false;
> >
> > @@ -1071,8 +1070,6 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
> >       if (unlikely(page->memcg_data))
> >               bad_reason = "page still charged to cgroup";
> >   #endif
> > -     if (unlikely(page_pool_page_is_pp(page)))
> > -             bad_reason = "page_pool leak";
> >       return bad_reason;
> >   }
> >
> > @@ -1381,9 +1378,17 @@ __always_inline bool __free_pages_prepare(struct page *page,
> >               mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
> >               folio->mapping = NULL;
> >       }
> > -     if (unlikely(page_has_type(page)))
> > +     if (unlikely(page_has_type(page))) {
> > +             /* networking expects to clear its page type before releasing */
> > +             if (is_check_pages_enabled()) {
> > +                     if (unlikely(PageNetpp(page))) {
> > +                             bad_page(page, "page_pool leak");
> > +                             return false;
> > +                     }
> > +             }
> >               /* Reset the page_type (which overlays _mapcount) */
> >               page->page_type = UINT_MAX;
> > +     }
>
> I need some opinions here.  When CONFIG_DEBUG_VM is enabled we get help
> finding (hard to find) page_pool leaks and mark the page as bad.
>

You mean we get debug messages about the leak right? Nothing is
silently freed with that config enabled.

> When CONFIG_DEBUG_VM is disabled we silently "allow" leaking.
> Should we handle this more gracefully here? (release pp resources)
>
> Allowing the page to be reused at this point, when a page_pool instance
> is known to track life-time and (likely) have the page DMA mapped, seems
> problematic to me.  Code only clears page->page_type, but IIRC Toke
> added DMA tracking in other fields (plus xarray internally).

If we can reliably track and release the pages, I think we should do
this regardless of CONFIG_DEBUG_VM being enabled or not. But the
question is how did that leak happen? Is it a misuse of the API from
the driver?

Thanks
/Ilias
>
> I was going to suggest to simply re-export page_pool_release_page() that
> was hidden by 535b9c61bdef ("net: page_pool: hide
> page_pool_release_page()"), but that functionality was lost in
> 07e0c7d3179d ("net: page_pool: merge page_pool_release_page() with
> page_pool_return_page()"). (Cc Jakub/Kuba for that choice).
> Simply page_pool_release_page(page->pp, page).
>
> Opinions?
>
> --Jesper