[PATCH v4 0/2] mm/uffd: Add feature bit UFFD_FEATURE_WP_UNPOPULATED

[Peter Xu <peterx@redhat.com>]

v1: https://lore.kernel.org/r/20230215210257.224243-1-peterx@redhat.com
v2: https://lore.kernel.org/r/20230227230044.1596744-1-peterx@redhat.com
v3: https://lore.kernel.org/r/20230306213925.617814-1-peterx@redhat.com

v4:
- s/handle_pte_missing/do_pte_missing/, fix spellings, etc. [David]
- Add a helper userfaultfd_wp_use_markers() [David]
- Update userfaultfd.rst describing the new feature bit

The new feature bit will make anonymous memory acts the same like file
memory on userfaultfd-wp in that it'll also wr-protect none ptes.

It can be useful in two cases:

(1) Uffd-wp app that needs to wr-protect none ptes like QEMU snapshot, so
    pre-fault can be replaced by enabling this flag and speed up protections

(2) It helps to implement async uffd-wp mode that Muhammad is working on [1]

It's debateable whether this is the most ideal solution because with the
new feature bit set, wr-protect none pte needs to pre-populate the pgtables
to the last level (PAGE_SIZE).  But it seems fine so far to service either
purpose above, so we can leave optimizations for later.

The series brings pte markers to anonymous memory too.  There's some change
in the common mm code path in the 1st patch, great to have some eye looking
at it, but hopefully they're still relatively straightforward.

Thanks,

[1] https://lore.kernel.org/all/Y+v2HJ8+3i%2FKzDBu@x1n/

Peter Xu (2):
  mm/uffd: UFFD_FEATURE_WP_UNPOPULATED
  selftests/mm: Smoke test UFFD_FEATURE_WP_UNPOPULATED

 Documentation/admin-guide/mm/userfaultfd.rst | 17 ++++++
 fs/userfaultfd.c                             | 16 ++++++
 include/linux/mm_inline.h                    |  6 +++
 include/linux/userfaultfd_k.h                | 23 ++++++++
 include/uapi/linux/userfaultfd.h             | 10 +++-
 mm/memory.c                                  | 56 +++++++++++++++-----
 mm/mprotect.c                                | 51 ++++++++++++++----
 tools/testing/selftests/mm/userfaultfd.c     | 45 +++++++++++++++-
 8 files changed, 197 insertions(+), 27 deletions(-)

-- 
2.39.1