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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/19/26 03:09, 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")

I would actually split that up into separate patches.

Over-indicating soft-dirty (as raised by Sashiko) is in general not a problem.

Losing WP bits is.

UFFD, the gift that keeps on giving.

How to deal RWP properly is a good question (see Sashiko review). I feel like
some more stuff might be missing there to handle this correctly.

-- 
Cheers,

David