[PATCH 1/7] mm: fix folio_expected_ref_count() when PG_private_2

[Hugh Dickins <hughd@google.com>]

6.16's folio_expected_ref_count() is forgetting the PG_private_2 flag,
which (like PG_private, but not in addition to PG_private) counts for
1 more reference: it needs to be using folio_has_private() in place of
folio_test_private().

But this went wrong earlier: folio_expected_ref_count() was based on
(and replaced) mm/migrate.c's folio_expected_refs(), which has been
using folio_test_private() since 6.0 converted to folios(): before
that, expected_page_refs() was correctly using page_has_private().

Just a few filesystems are still using PG_private_2 a.k.a. PG_fscache.
Potentially, this fix re-enables page migration on their folios; but
it would not be surprising to learn that in practice those folios are
not migratable for other reasons.

Fixes: 86ebd50224c0 ("mm: add folio_expected_ref_count() for reference count calculation")
Fixes: 108ca8358139 ("mm/migrate: Convert expected_page_refs() to folio_expected_refs()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
 include/linux/mm.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..ee8e535eadac 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2180,8 +2180,8 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 	} else {
 		/* One reference per page from the pagecache. */
 		ref_count += !!folio->mapping << order;
-		/* One reference from PG_private. */
-		ref_count += folio_test_private(folio);
+		/* One reference from PG_private or PG_private_2. */
+		ref_count += folio_has_private(folio);
 	}
 
 	/* One reference per page table mapping. */
-- 
2.51.0