Patch series " JFS: Implement migrate_folio for jfs_metapage_aops" v5.

This patchset addresses a warning that occurs during memory compaction due to JFS's missing migrate_folio operation. The warning was introduced by commit 7ee3647243e5 ("migrate: Remove call to ->writepage") which added explicit warnings when filesystem don't implement migrate_folio.

The syzbot reported following [1]: jfs_metapage_aops does not implement migrate_folio WARNING: CPU: 1 PID: 5861 at mm/migrate.c:955 fallback_migrate_folio mm/migrate.c:953 [inline] WARNING: CPU: 1 PID: 5861 at mm/migrate.c:955 move_to_new_folio+0x70e/0x840 mm/migrate.c:1007 Modules linked in: CPU: 1 UID: 0 PID: 5861 Comm: syz-executor280 Not tainted 6.15.0-rc1-next-20250411-syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025 RIP: 0010:fallback_migrate_folio mm/migrate.c:953 [inline] RIP: 0010:move_to_new_folio+0x70e/0x840 mm/migrate.c:1007

To fix this issue, this series implement metapage_migrate_folio() for JFS which handles both single and multiple metapages per page configurations.

While most filesystems leverage existing migration implementations like filemap_migrate_folio(), buffer_migrate_folio_norefs() or buffer_migrate_folio() (which internally used folio_expected_refs()), JFS's metapage architecture requires special handling of its private data during migration. To support this, this series introduce the folio_expected_ref_count(), which calculates external references to a folio from page/swap cache, private data, and page table mappings.

This standardized implementation replaces the previous ad-hoc folio_expected_refs() function and enables JFS to accurately determine whether a folio has unexpected references before attempting migration.

Implement folio_expected_ref_count() to calculate expected folio reference counts from: - Page/swap cache (1 per page) - Private data (1) - Page table mappings (1 per map)

While originally needed for page migration operations, this improved implementation standardizes reference counting by consolidating all refcount contributors into a single, reusable function that can benefit any subsystem needing to detect unexpected references to folios.

The folio_expected_ref_count() returns the sum of these external references without including any reference the caller itself might hold. Callers comparing against the actual folio_ref_count() must account for their own references separately.

Link: https://syzkaller.appspot.com/bug?extid=8bb6fd945af4e0ad9299 [1] Link: https://lkml.kernel.org/r/20250430100150.279751-1-shivankg@amd.com Link: https://lkml.kernel.org/r/20250430100150.279751-2-shivankg@amd.com Signed-off-by: David Hildenbrand david@redhat.com Signed-off-by: Shivank Garg shivankg@amd.com Suggested-by: Matthew Wilcox willy@infradead.org Co-developed-by: David Hildenbrand david@redhat.com Cc: Alistair Popple apopple@nvidia.com Cc: Dave Kleikamp shaggy@kernel.org Cc: Donet Tom donettom@linux.ibm.com Cc: Jane Chu jane.chu@oracle.com Cc: Kefeng Wang wangkefeng.wang@huawei.com Cc: Zi Yan ziy@nvidia.com Signed-off-by: Andrew Morton akpm@linux-foundation.org --- include/linux/mm.h | 55 ++++++++++++++++++++++++++++++++++++++++++++++ mm/migrate.c | 22 ++++--------------- 2 files changed, 59 insertions(+), 18 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h index e51dba8398f7..000a67a789ad 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2306,6 +2306,61 @@ static inline bool folio_maybe_mapped_shared(struct folio *folio) return folio_test_large_maybe_mapped_shared(folio); }

+/** + * folio_expected_ref_count - calculate the expected folio refcount + * @folio: the folio + * + * Calculate the expected folio refcount, taking references from the pagecache, + * swapcache, PG_private and page table mappings into account. Useful in + * combination with folio_ref_count() to detect unexpected references (e.g., + * GUP or other temporary references). + * + * Does currently not consider references from the LRU cache. If the folio + * was isolated from the LRU (which is the case during migration or split), + * the LRU cache does not apply. + * + * Calling this function on an unmapped folio -- !folio_mapped() -- that is + * locked will return a stable result. + * + * Calling this function on a mapped folio will not result in a stable result, + * because nothing stops additional page table mappings from coming (e.g., + * fork()) or going (e.g., munmap()). + * + * Calling this function without the folio lock will also not result in a + * stable result: for example, the folio might get dropped from the swapcache + * concurrently. + * + * However, even when called without the folio lock or on a mapped folio, + * this function can be used to detect unexpected references early (for example, + * if it makes sense to even lock the folio and unmap it). + * + * The caller must add any reference (e.g., from folio_try_get()) it might be + * holding itself to the result. + * + * Returns the expected folio refcount. + */ +static inline int folio_expected_ref_count(const struct folio *folio) +{ + const int order = folio_order(folio); + int ref_count = 0; + + if (WARN_ON_ONCE(folio_test_slab(folio))) + return 0; + + if (folio_test_anon(folio)) { + /* One reference per page from the swapcache. */ + ref_count += folio_test_swapcache(folio) << order; + } else if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS)) { + /* 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 per page table mapping. */ + return ref_count + folio_mapcount(folio); +} + #ifndef HAVE_ARCH_MAKE_FOLIO_ACCESSIBLE static inline int arch_make_folio_accessible(struct folio *folio) { diff --git a/mm/migrate.c b/mm/migrate.c index 676d9cfc7059..273d46771a6c 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -445,20 +445,6 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd) } #endif

-static int folio_expected_refs(struct address_space *mapping, - struct folio *folio) -{ - int refs = 1; - if (!mapping) - return refs; - - refs += folio_nr_pages(folio); - if (folio_test_private(folio)) - refs++; - - return refs; -} - /* * Replace the folio in the mapping. * @@ -601,7 +587,7 @@ static int __folio_migrate_mapping(struct address_space *mapping, int folio_migrate_mapping(struct address_space *mapping, struct folio *newfolio, struct folio *folio, int extra_count) { - int expected_count = folio_expected_refs(mapping, folio) + extra_count; + int expected_count = folio_expected_ref_count(folio) + extra_count + 1;

if (folio_ref_count(folio) != expected_count) return -EAGAIN; @@ -618,7 +604,7 @@ int migrate_huge_page_move_mapping(struct address_space *mapping, struct folio *dst, struct folio *src) { XA_STATE(xas, &mapping->i_pages, folio_index(src)); - int rc, expected_count = folio_expected_refs(mapping, src); + int rc, expected_count = folio_expected_ref_count(src) + 1;

if (folio_ref_count(src) != expected_count) return -EAGAIN; @@ -749,7 +735,7 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst, struct folio *src, void *src_private, enum migrate_mode mode) { - int rc, expected_count = folio_expected_refs(mapping, src); + int rc, expected_count = folio_expected_ref_count(src) + 1;

/* Check whether src does not have extra refs before we do more work */ if (folio_ref_count(src) != expected_count) @@ -837,7 +823,7 @@ static int __buffer_migrate_folio(struct address_space *mapping, return migrate_folio(mapping, dst, src, mode);

/* Check whether page does not have extra refs before we do more work */ - expected_count = folio_expected_refs(mapping, src); + expected_count = folio_expected_ref_count(src) + 1; if (folio_ref_count(src) != expected_count) return -EAGAIN;