On Fri, Jun 20, 2025 at 10:28:16AM +0000, Shivank Garg wrote:
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(-)
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This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly.
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