[PATCH v1 2/2] mm/rmap: sanity check that zeropages are not passed to RMAP

[David Hildenbrand <david@redhat.com>]

Using insert_page() we might have previously ended up passing the zeropage
into rmap code. Make sure that won't happen again.

Note that we won't check the huge zeropage for now, which might still
end up in RMAP code.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 include/linux/rmap.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index 7229b9baf20d8..5cb0d419a1d75 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -200,6 +200,9 @@ static inline void __folio_rmap_sanity_checks(struct folio *folio,
 	/* hugetlb folios are handled separately. */
 	VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 
+	/* When (un)mapping zeropages, we should never touch ref+mapcount. */
+	VM_WARN_ON_FOLIO(is_zero_folio(folio), folio);
+
 	/*
 	 * TODO: we get driver-allocated folios that have nothing to do with
 	 * the rmap using vm_insert_page(); therefore, we cannot assume that
-- 
2.44.0