On Tue, Apr 09, 2024 at 09:22:44PM +0200, David Hildenbrand wrote:

Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
pages") made it impossible to detect mapcount underflows by treating
any negative raw mapcount value as a mapcount of 0.

Yes, but I don't think this is the right place to check for underflow.
We should be checking for that on modification, not on read.  I think
it's more important for page_mapcount() to be fast than a debugging aid.