mm/memory: use folio_mapcount() in zap_present_folio_ptes()

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.  In zap_present_folio_ptes(), let's simply check the
folio mapcount().  If there is some issue, it will underflow at some point
either way when unmapping.

As indicated already in commit 10ebac4f95 ("mm/memory: optimize
unmap/zap with PTE-mapped THP"), we already documented "If we ever have a
cheap folio_mapcount(), we might just want to check for underflows
there.".

There is no change for small folios.  For large folios, we'll now catch
more underflows when batch-unmapping, because instead of only testing the
mapcount of the first subpage, we'll test if the folio mapcount
underflows.

Link: https://lkml.kernel.org/r/20240409192301.907377-8-david@redhat.com
Signed-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/memory.c

@@ -1502,8 +1502,7 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
 	if (!delay_rmap) {
 		folio_remove_rmap_ptes(folio, page, nr, vma);
 
-		/* Only sanity-check the first page in a batch. */
-		if (unlikely(page_mapcount(page) < 0))
+		if (unlikely(folio_mapcount(folio) < 0))
 			print_bad_pte(vma, addr, ptent, page);
 	}