Re: [PATCH 6.12.y 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

[Lance Yang <lance.yang@linux.dev>]

On 2025/10/17 17:52, Lorenzo Stoakes wrote:
> On Fri, Oct 17, 2025 at 04:51:06PM +0800, 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.
>>
>> Link: https://lkml.kernel.org/r/20250930081040.80926-1-lance.yang@linux.dev
>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> 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>
>> Acked-by: Zi Yan <ziy@nvidia.com>
>> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
>> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> 
> You're missing my R-b...

Sorry, I missed it! I just cherry-picked the commit from
upstream and didn't notice ...

Hopefully Greg can add your Reviewed-by when applying.