On 30.06.25 17:27, Lorenzo Stoakes wrote:

On Mon, Jun 30, 2025 at 02:59:45PM +0200, David Hildenbrand wrote:

quoted

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.

quoted

Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Harry Yoo <redacted>
Signed-off-by: David Hildenbrand <redacted>
---
  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