mm/huge_memory: use folio_mapcount() in zap_huge_pmd() sanity check

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.  Let's similarly check for folio_mapcount()
underflows instead of page_mapcount() underflows like we do in
zap_present_folio_ptes() now.

Instead of the VM_BUG_ON(), we should actually be doing something like
print_bad_pte().  For now, let's keep it simple and use WARN_ON_ONCE(),
performing that check independently of DEBUG_VM.

Link: https://lkml.kernel.org/r/20240409192301.907377-9-david@redhat.comSigned-off-by: David Hildenbrand <david@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Rich Felker <dalias@libc.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/huge_memory.c

@@ -1791,7 +1791,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 
 			folio = page_folio(page);
 			folio_remove_rmap_pmd(folio, page, vma);
-			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
+			WARN_ON_ONCE(folio_mapcount(folio) < 0);
 			VM_BUG_ON_PAGE(!PageHead(page), page);
 		} else if (thp_migration_supported()) {
 			swp_entry_t entry;