Re: [PATCH v1 04/29] mm/page_alloc: let page freeing clear any set page type

[David Hildenbrand]

On 30.06.25 17:27, Lorenzo Stoakes wrote:
> On Mon, Jun 30, 2025 at 02:59:45PM +0200, David Hildenbrand wrote:
> > Currently, any user of page types must clear that type before freeing
> > a page back to the buddy, otherwise we'll run into mapcount related
> > sanity checks (because the page type currently overlays the page
> > mapcount).
> >
> > Let's allow for not clearing the page type by page type users by letting
> > the buddy handle it instead.
> >
> > We'll focus on having a page type set on the first page of a larger
> > allocation only.
> >
> > With this change, we can reliably identify typed folios even though
> > they might be in the process of getting freed, which will come in handy
> > in migration code (at least in the transition phase).
> >
> > In the future we might want to warn on some page types. Instead of
> > having an "allow list", let's rather wait until we know about once that
> > should go on such a "disallow list".
>
> Is the idea here to get this to show up on folio dumps or?

As part of the netmem_desc series, there was a discussion about removing 
the mystical PP checks -- page_pool_page_is_pp() in page_alloc.c and 
replacing them by a proper page type check.

In that case, we would probably want to warn in case we get such a 
netmem page unexpectedly freed.

But, that page type does not exist yet in code, so the sanity check must 
be added once introduced.

> > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> > Acked-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >   mm/page_alloc.c | 3 +++
> >   1 file changed, 3 insertions(+)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 858bc17653af9..44e56d31cfeb1 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1380,6 +1380,9 @@ __always_inline bool free_pages_prepare(struct page *page,
> >   			mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1);
> >   		page->mapping = NULL;
> >   	}
> > +	if (unlikely(page_has_type(page)))
> > +		page->page_type = UINT_MAX;
>
> Feels like this could do with a comment!

/* Reset the page_type -> _mapcount to -1 */

--
Cheers,

David / dhildenb