linux-kernel - [PATCH 1/7] mm: fix folio_expected_ref_count() when PG_private

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Message-ID: <f91ee36e-a8cb-e3a4-c23b-524ff3848da7@google.com>
Date: Sun, 31 Aug 2025 02:01:16 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: David Hildenbrand <david@...hat.com>, Shivank Garg <shivankg@....com>, 
    Matthew Wilcox <willy@...radead.org>, 
    Christoph Hellwig <hch@...radead.org>, Keir Fraser <keirf@...gle.com>, 
    Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>, 
    Frederick Mayle <fmayle@...gle.com>, Peter Xu <peterx@...hat.com>, 
    "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, 
    Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>, 
    Alexander Krabler <Alexander.Krabler@...a.com>, 
    Ge Yang <yangge1116@....com>, Li Zhe <lizhe.67@...edance.com>, 
    Chris Li <chrisl@...nel.org>, Yu Zhao <yuzhao@...gle.com>, 
    Axel Rasmussen <axelrasmussen@...gle.com>, 
    Yuanchu Xie <yuanchu@...gle.com>, Wei Xu <weixugc@...gle.com>, 
    Konstantin Khlebnikov <koct9i@...il.com>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: [PATCH 1/7] mm: fix folio_expected_ref_count() when PG_private_2

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@...gle.com>
Cc: <stable@...r.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