Re: [PATCH v2 09/14] mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP

[Mike Rapoport <rppt@kernel.org>]

On Fri, May 08, 2026 at 04:55:21PM +0100, Kiryl Shutsemau (Meta) wrote:
> Wire the fault side of read-write protection tracking and turn the
> userspace interface on.
> 
> An RWP-protected PTE is PAGE_NONE with the uffd bit set. The
> PROT_NONE triggers a fault on any access; the uffd bit distinguishes
> it from plain mprotect(PROT_NONE) or NUMA hinting.
> 
> Fault dispatch, per level:
> 
>   PTE     handle_pte_fault()    -> do_uffd_rwp()
>   PMD     __handle_mm_fault()   -> do_huge_pmd_uffd_rwp()
>   hugetlb hugetlb_fault()       -> hugetlb_handle_userfault()
> 
> The RWP branches gate on userfaultfd_pte_rwp() / userfaultfd_huge_pmd_rwp()
> (VM_UFFD_RWP plus the uffd bit) and fall through to do_numa_page() /
> do_huge_pmd_numa_page() otherwise. Each delivers a
> UFFD_PAGEFAULT_FLAG_RWP message through handle_userfault(); the handler
> resolves it with UFFDIO_RWPROTECT clearing MODE_RWP.
> 
> userfaultfd_must_wait() and userfaultfd_huge_must_wait() add matching
> protnone+uffd waiters so sync-mode fault handlers block correctly.
> 
> Expose the UAPI:
> 
>   UFFDIO_REGISTER_MODE_RWP   -> UFFD_API_REGISTER_MODES
>   UFFD_FEATURE_RWP           -> UFFD_API_FEATURES
>   _UFFDIO_RWPROTECT          -> UFFD_API_RANGE_IOCTLS
>                                 UFFD_API_RANGE_IOCTLS_BASIC
> 
> UFFD_FEATURE_RWP is masked out at UFFDIO_API time when PROT_NONE is
> not available or VM_UFFD_RWP aliases VM_NONE (32-bit), so userspace
> never sees an advertised-but-broken feature.
> 
> Works on anonymous, shmem, and hugetlb memory.
> 
> Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> Assisted-by: Claude:claude-opus-4-6

A small nit below, other than that

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> @@ -347,6 +359,14 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
>  	 */
>  	if (!pte_write(ptent) && (reason & VM_UFFD_WP))
>  		goto out;
> +	/*
> +	 * PTE is still RW-protected (protnone with uffd bit), wait for
> +	 * userspace to resolve. Plain PROT_NONE without the marker is not
> +	 * an RWP fault.
> +	 */
> +	if (pte_protnone(ptent) && pte_uffd(ptent) &&
> +	    (reason & VM_UFFD_RWP))

Nit: this fits even in 80-chars line

> +		goto out;
>  
>  	ret = false;
>  out:

-- 
Sincerely yours,
Mike.