On 30 Sep 2025, at 4:10, Lance Yang wrote:
From: Lance Yang lance.yang@linux.dev
When splitting an mTHP and replacing a zero-filled subpage with the shared zeropage, try_to_map_unused_to_zeropage() currently drops several important PTE bits.
For userspace tools like CRIU, which rely on the soft-dirty mechanism for incremental snapshots, losing the soft-dirty bit means modified pages are missed, leading to inconsistent memory state after restore.
As pointed out by David, the more critical uffd-wp bit is also dropped. This breaks the userfaultfd write-protection mechanism, causing writes to be silently missed by monitoring applications, which can lead to data corruption.
Preserve both the soft-dirty and uffd-wp bits from the old PTE when creating the new zeropage mapping to ensure they are correctly tracked.
Cc: stable@vger.kernel.org Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Suggested-by: David Hildenbrand david@redhat.com Suggested-by: Dev Jain dev.jain@arm.com Acked-by: David Hildenbrand david@redhat.com Reviewed-by: Dev Jain dev.jain@arm.com Signed-off-by: Lance Yang lance.yang@linux.dev
v4 -> v5:
- Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped THP migration entries.
- https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.dev...
v3 -> v4:
- Minor formatting tweak in try_to_map_unused_to_zeropage() function signature (per David and Dev)
- Collect Reviewed-by from Dev - thanks!
- https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev...
v2 -> v3:
- ptep_get() gets called only once per iteration (per Dev)
- https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev...
v1 -> v2:
- Avoid calling ptep_get() multiple times (per Dev)
- Double-check the uffd-wp bit (per David)
- Collect Acked-by from David - thanks!
- https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev...
mm/migrate.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-)
Acked-by: Zi Yan ziy@nvidia.com
Best Regards, Yan, Zi