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
