Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

[Harry Yoo]

On Tue, Sep 30, 2025 at 04:10:40PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@xxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxxx>
> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Dev Jain <dev.jain@xxxxxxx>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Dev Jain <dev.jain@xxxxxxx>
> Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@xxxxxxxxxx>

-- 
Cheers,
Harry / Hyeonggon