Re: [PATCH 3/3] alloc_tag: avoid warnings when freeing non-compound "tail" pages

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Sep 11, 2025 at 1:01 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Tue, Sep 09, 2025 at 04:34:09PM -0700, Suren Baghdasaryan wrote:
> > When freeing "tail" pages of a non-compount high-order page, we properly
> > subtract the allocation tag counters, however later when these pages are
> > released, alloc_tag_sub() will issue warnings because tags for these pages
> > are NULL.
> > This issue was originally anticipated by Vlastimil in his review [1] and
> > then recently reported by David.
> > Prevent warnings by marking the tags empty.
> >
> > [1] https://lore.kernel.org/all/6db0f0c8-81cb-4d04-9560-ba73d63db4b8@suse.cz/
> >
> > Suggested-by: David Wang <00107082@163.com>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/page_alloc.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 1760346bbd24..d21a411e807e 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5240,9 +5240,16 @@ static void ___free_pages(struct page *page, unsigned int order,
> >               __free_frozen_pages(page, order, fpi_flags);
> >       else if (!head) {
> >               pgalloc_tag_sub_pages(tag, (1 << order) - 1);
> > -             while (order-- > 0)
> > +             while (order-- > 0) {
> > +                     /*
> > +                      * The "tail" pages of this non-compound high-order
> > +                      * page will have no code tags, so to avoid warnings
> > +                      * mark them as empty.
> > +                      */
> > +                     clear_page_tag_ref(page + (1 << order));
>
> Do we need something similar for the release_pages() code path or is it
> happening already?

Pages released with release_pages() should have valid tags, so I
expect no warnings.

>
> >                       __free_frozen_pages(page + (1 << order), order,
> >                                           fpi_flags);
> > +             }
> >       }
> >  }
> >
> > --
> > 2.51.0.384.g4c02a37b29-goog
> >
>