[PATCH V5 0/2] JFS: Implement migrate_folio for jfs_metapage_aops

[Shivank Garg <shivankg@amd.com>]

This patch 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.

[1]: https://syzkaller.appspot.com/bug?extid=8bb6fd945af4e0ad9299

Changelogs:
V5 (current):
- Add folio_expected_ref_count() for refcount calculation
- Add details about need of folio_expected_ref_count() for JFS

V4:
- https://lore.kernel.org/all/20250422114000.15003-1-shivankg@amd.com
- Make folio_expected_refs() inline and rename to folio_migration_expected_refs()

V3:
- https://lore.kernel.org/all/20250417060630.197278-1-shivankg@amd.com
- Fix typos

V1/V2:
- https://lore.kernel.org/all/20250413172356.561544-1-shivankg@amd.com
- Implement metapage_migrate_folio() similar to buffer_migrate_folio() but
  specialized to move JFS metapage data

#syz test: https://github.com/shivankgarg98/linux.git 69a58d5260

Shivank Garg (2):
  mm: Add folio_expected_ref_count() for reference count calculation
  jfs: implement migrate_folio for jfs_metapage_aops

 fs/jfs/jfs_metapage.c | 94 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h    | 55 +++++++++++++++++++++++++
 mm/migrate.c          | 22 ++--------
 3 files changed, 153 insertions(+), 18 deletions(-)

-- 
2.34.1