mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP

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.

Link: https://lore.kernel.org/20260708111417.173443-11-kirill@shutemov.name
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions