Re: [PATCH v2] mm/ksm: keep soft-dirty and uffd-wp when merging a page

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

On 2026/10/19 09:09, Samarth Narang wrote:
> replace_page() maps the KSM page with a PTE built from scratch, so the
> soft-dirty and uffd bits of the PTE it replaces are lost. Due to this, a
> write before the merge no longer shows as soft-dirty, the next write to
> a uffd-wp page raises no write-protect fault, and because the page is
> mapped with the VMA's normal permissions, on a uffd-rwp VMA the next
> access raises no RWP fault.
> 
> Copy the soft-dirty and uffd bits from the original PTE to the new PTE,
> and restore PAGE_NONE for RWP VMAs, as try_to_map_unused_to_zeropage()
> does.
> 
> Fixes: 0f8975ec4db2 ("mm: soft-dirty bits for user memory changes tracking")
> Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
> Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP")
> Cc: stable@vger.kernel.org
> Tested-by: Lance Yang <lance.yang@linux.dev>
> Assisted-by: LLM
> Signed-off-by: Samarth Narang <samarth.colleges@gmail.com>
> ---
> 
> Notes:
>      v1: https://lore.kernel.org/all/20261018122957.526467-1-samarth.colleges@gmail.com/
>      
>      Changes since v1 (thanks, Lance):
>      - Fixes: point the uffd-wp and RWP tags at the commits that make the
>        registration modes reachable (63b2d4174c4a, 2d427bb01639).
>      - Shorten the comment; the uffd bit carries RWP state too.
>      - Add Lance's Tested-by.

Generally, no need to rush the next version out, please. Let it soak for a
few days so folks have time to review properly, instead of making everyone
start over every time. Just saying :)

Cheers, Lance