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

[Zi Yan]

On 30 Sep 2025, at 4:10, 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>
> ---
> 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@xxxxxxxxx/
>
> 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@xxxxxxxxx/
>
> v2 -> v3:
>  - ptep_get() gets called only once per iteration (per Dev)
>  - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@xxxxxxxxx/
>
> 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@xxxxxxxxx/
>
>  mm/migrate.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>

Acked-by: Zi Yan <ziy@xxxxxxxxxx>


Best Regards,
Yan, Zi