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".

Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Harry Yoo <redacted>
Reviewed-by: Lorenzo Stoakes <redacted>
Signed-off-by: David Hildenbrand <redacted>
---
 mm/page_alloc.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 858bc17653af9..b825f224af01f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -1380,6 +1380,10 @@ __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)))
+		/* Reset the page_type (which overlays _mapcount) */
+		page->page_type = UINT_MAX;
+
 	if (is_check_pages_enabled()) {
 		if (free_page_is_bad(page))
 			bad++;

-- 
2.49.0