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

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

On Fri, 20 Mar 2026 at 13:44, Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>
>
>
> On 18/03/2026 03.02, Byungchul Park wrote:
> > On Tue, Mar 17, 2026 at 10:20:08AM +0100, Jesper Dangaard Brouer 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.
> >>
> >> 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).
> >>
> >> 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?
> >
> > This is not an issue related to this patch but it's worth discussing. :)
> >
>
> Yes, looking at existing code, you are actually keeping the same
> problematic semantics of freeing a page that is still tracked by a
> page_pool (DMA-mapped and life-time tracked), unless CONFIG_DEBUG_VM is
> enabled.  You code change just makes this more clear that this is the case.
>
> In that sense this patch is correct and can be applied.
> Let me clearly ACK this patch.
>
> Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

>
>
> > Not only page pool state but any bad states checks are also skipped with
> > is_check_pages_enabled() off, which means they are going to be
> > suppressed with the off.
> >
>
> Sadly yes. I'm mostly familiar with the page_pool, and I can say that
> this isn't a good situation.  Like Dragos said [1] it would be better to
> just leak to it.  Dragos gave a talk[2] on how to find these leaked
> pages, if we let them stay leaked.
>
>
>   [1] https://lore.kernel.org/all/20260319163109.58511b70@kernel.org/
>   [2]
> https://netdevconf.info/0x19/sessions/tutorial/diagnosing-page-pool-leaks.html
>
>
> > It is necessary to first clarify whether the action was intentional or
> > not.  According to the answer, we can discuss what to do better. :)
> >
> Acking this patch, as this discussion is a problem for "us" (netdev
> people) to figure out.  Lets land this patch and then we can followup.
>
> --Jesper
>
>
>
>