[PATCH v1 07/18] mm/memory: use folio_mapcount() in zap_present_folio_ptes()

[David Hildenbrand]

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 10ebac4f95e7 ("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.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/memory.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 78422d1c7381..178492efb4af 100644
--- a/mm/memory.c
+++ b/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);
 	}
 
-- 
2.44.0