6.6-stable review patch. If anyone has any objections, please let me know.
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From: Shivank Garg shivankg@amd.com
[ Upstream commit 86ebd50224c0734d965843260d0dc057a9431c61 ]
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 Stable-dep-of: 98c6d259319e ("mm/gup: check ref_count instead of lru before migration") [ Take the new function in mm.h, removing "const" from its parameter to stop build warnings; but avoid all the conflicts of using it in mm/migrate.c. ] Signed-off-by: Hugh Dickins hughd@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/mm.h | 55 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+)
diff --git a/include/linux/mm.h b/include/linux/mm.h index b97d8a691b28b..ba77f08900ca2 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2156,6 +2156,61 @@ static inline int folio_estimated_sharers(struct folio *folio) return page_mapcount(folio_page(folio, 0)); }
+/** + * 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(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_PAGE_ACCESSIBLE static inline int arch_make_page_accessible(struct page *page) {