The patch titled Subject: mm: fix folio_expected_ref_count() when PG_private_2 has been added to the -mm mm-new branch. Its filename is mm-fix-folio_expected_ref_count-when-pg_private_2.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Hugh Dickins hughd@google.com Subject: mm: fix folio_expected_ref_count() when PG_private_2 Date: Sun, 31 Aug 2025 02:01:16 -0700 (PDT)

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.

Link: https://lkml.kernel.org/r/f91ee36e-a8cb-e3a4-c23b-524ff3848da7@google.com 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: "Aneesh Kumar K.V" aneesh.kumar@kernel.org Cc: Axel Rasmussen axelrasmussen@google.com Cc: Chris Li chrisl@kernel.org Cc: Christoph Hellwig hch@infradead.org Cc: David Hildenbrand david@redhat.com Cc: Jason Gunthorpe jgg@ziepe.ca Cc: Johannes Weiner hannes@cmpxchg.org Cc: John Hubbard jhubbard@nvidia.com Cc: Keir Fraser keirf@google.com Cc: Konstantin Khlebnikov koct9i@gmail.com Cc: Li Zhe lizhe.67@bytedance.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Peter Xu peterx@redhat.com Cc: Shivank Garg shivankg@amd.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Wei Xu weixugc@google.com Cc: Will Deacon will@kernel.org Cc: yangge yangge1116@126.com Cc: Yuanchu Xie yuanchu@google.com Cc: Yu Zhao yuzhao@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/mm.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h~mm-fix-folio_expected_ref_count-when-pg_private_2 +++ a/include/linux/mm.h @@ -2234,8 +2234,8 @@ static inline int folio_expected_ref_cou } 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. */ _

Patches currently in -mm which might be from hughd@google.com are

mm-fix-folio_expected_ref_count-when-pg_private_2.patch mm-gup-check-ref_count-instead-of-lru-before-migration.patch mm-gup-local-lru_add_drain-to-avoid-lru_add_drain_all.patch mm-revert-mm-gup-clear-the-lru-flag-of-a-page-before-adding-to-lru-batch.patch mm-revert-mm-vmscanc-fix-oom-on-swap-stress-test.patch mm-folio_may_be_cached-unless-folio_test_large.patch mm-lru_add_drain_all-do-local-lru_add_drain-first.patch