Re: [PATCH v2 08/14] userfaultfd: add UFFDIO_REGISTER_MODE_RWP and UFFDIO_RWPROTECT plumbing

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, May 12, 2026 at 08:20:14PM +0300, Mike Rapoport wrote:
> On Fri, May 08, 2026 at 04:55:20PM +0100, Kiryl Shutsemau (Meta) wrote:
> > Add the userspace interface for read-write protection tracking:
> > 
> >   - UFFDIO_REGISTER_MODE_RWP      register a range for RWP tracking
> >   - UFFD_FEATURE_RWP              capability bit
> >   - UFFDIO_RWPROTECT              install / remove RWP on a range
> > 
> > Registration sets VM_UFFD_RWP on the VMA. Combining MODE_WP with
> > MODE_RWP is rejected because both modes claim the uffd PTE bit.
> > 
> > UFFDIO_RWPROTECT is the bidirectional counterpart of
> > UFFDIO_WRITEPROTECT:
> > 
> >   - MODE_RWP              change_protection() with MM_CP_UFFD_RWP
> >                           installs PAGE_NONE and sets the uffd bit on
> >                           present PTEs
> >   - !MODE_RWP             change_protection() with MM_CP_UFFD_RWP_RESOLVE
> >                           restores vma->vm_page_prot and clears the bit
> > 
> > userfaultfd_clear_vma() runs the same resolve pass on unregister so
> > RWP state cannot outlive the uffd.
> > 
> > Re-registering a range must not drop a mode that installs per-PTE
> > markers (WP or RWP); doing so returns -EBUSY. This also closes a
> > pre-existing window where re-registering without MODE_WP would strand
> > uffd-wp markers: before, those caused extra write-faults but were
> > otherwise benign; with RWP preservation in place, a subsequent
> > mprotect() on a VM_UFFD_RWP VMA would silently promote the stale
> > markers to RWP.
> > 
> > The feature is not yet advertised. UFFDIO_REGISTER_MODE_RWP,
> > UFFD_FEATURE_RWP, and _UFFDIO_RWPROTECT are intentionally absent from
> > UFFD_API_REGISTER_MODES, UFFD_API_FEATURES, and UFFD_API_RANGE_IOCTLS,
> > so UFFDIO_API masks them out and the register-mode validator rejects
> > the bit. The follow-up patch adds fault dispatch and exposes the UAPI.
> > 
> > Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
> > Assisted-by: Claude:claude-opus-4-6
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

Thanks!

> 
> with a comment below
> 
> > ---
> >  Documentation/admin-guide/mm/userfaultfd.rst | 10 ++
> >  fs/userfaultfd.c                             | 84 +++++++++++++++++
> >  include/linux/userfaultfd_k.h                |  2 +
> >  include/uapi/linux/userfaultfd.h             | 19 ++++
> >  mm/userfaultfd.c                             | 97 +++++++++++++++++++-
> >  5 files changed, 209 insertions(+), 3 deletions(-)
> > 
> > +	/*
> > +	 * Pre-scan the range: validate every spanned VMA before applying
> > +	 * any change_protection() so a partial failure cannot leave the
> > +	 * process with only a prefix of the range re-protected.
> > +	 */
> > +	err = -ENOENT;
> > +	for_each_vma_range(vmi, dst_vma, end) {
> > +		if (!userfaultfd_rwp(dst_vma))
> > +			return -ENOENT;
> > +
> > +		if (is_vm_hugetlb_page(dst_vma)) {
> > +			unsigned long page_mask;
> > +
> > +			page_mask = vma_kernel_pagesize(dst_vma) - 1;
> > +			if ((start & page_mask) || (len & page_mask))
> > +				return -EINVAL;
> > +		}
> > +		err = 0;
> > +	}
> > +	if (err)
> > +		return err;
> 
> It's an interesting way to say "no VMA found in range" :)
> I think bool found and
> 	
> 	if (!found)
> 		return -ENOENT;
> 
> looks more readable.

Fair enough. Will do.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov