Re: [PATCH v4 05/14] mm: add MM_CP_UFFD_RWP change_protection() flag

[Kiryl Shutsemau]

On Mon, May 25, 2026 at 12:13:18PM +0000, sashiko-bot@xxxxxxxxxx wrote:
> Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
> 
> New issues:
> - [High] UFFD_RWP silently fails to apply PAGE_NONE protection on
> architectures without CONFIG_ARCH_HAS_PTE_PROTNONE.

CONFIG_USERFAULTFD_RWP depends on ARCH_HAS_PTE_PROTNONE (mm/Kconfig)
and VM_UFFD_RWP is VM_NONE without it (include/linux/mm.h), so
UFFDIO_REGISTER_MODE_RWP is rejected and change_protection() is never
called with MM_CP_UFFD_RWP on such archs.

> - [High] Resolving UFFD_RWP incorrectly clears UFFD_WP PTE markers.

WP and RWP are mutually exclusive at registration (the explicit reject
in userfaultfd_register()), and UFFDIO_SET_MODE only toggles RWP_ASYNC,
not the WP/RWP mode itself. A VMA armed with one can't carry the
other's markers; the (uffd_wp_resolve || uffd_rwp_resolve) OR is safe.

> 
> Pre-existing issues:
> - [High] mprotect() silently drops userfaultfd protection bits from
> migration and device-private swap entries.

Confirmed and pre-existing -- change_softleaf_pte() preserves
pte_swp_soft_dirty but not pte_swp_uffd when rebuilding a migration
or device_private_write entry. Plain mprotect() over a migrating page
strips the bit, breaking the trap on swap-in. Will address in a
separate fix after this series.

> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260525113737.1942478-1-kas@xxxxxxxxxx?part=5

-- 
  Kiryl Shutsemau / Kirill A. Shutemov