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 18:25, Lance Yang wrote:
> 
> 
> 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.

Looking at the timeline again, the fix was actually merged
upstream before your review arrived, so the commit I
cherry-picked never had your tag to begin with :(

Still hoping Greg can add it!