[PATCH v7 0/8] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

This series extends the HMM framework to support userfaultfd-backed memory
by allowing the mmap read lock to be dropped during hmm_range_fault().

Some page fault handlers — most notably userfaultfd — require the mmap lock
to be released so that userspace can resolve the fault. The current HMM
interface never sets FAULT_FLAG_ALLOW_RETRY, making it impossible to fault
in pages from userfaultfd-registered regions.

This series follows the established int *locked pattern from
get_user_pages_remote() in mm/gup.c. A new helper function,
hmm_range_fault_locked(), accepts an int *locked parameter. When the
mmap lock is dropped during fault resolution (VM_FAULT_RETRY or
VM_FAULT_COMPLETED), the function returns 0 with *locked = 0, signalling
the caller to restart its walk. The existing hmm_range_fault() is
refactored into a thin wrapper that passes NULL, preserving current
behavior for all existing callers.

Possible approaches to lift this limitation are documented in
Documentation/mm/hmm.rst.

Changes in v7:
  - Replaced the unlocked HMM API with
    hmm_range_fault_unlocked_timeout(). The helper now takes a timeout in
    jiffies, with 0 meaning retry indefinitely.
  - Moved -EBUSY retry handling into the HMM helper for the unlocked path.
    The helper refreshes range->notifier_seq internally before each retry.
  - Switched the unlocked path to mmap_read_lock_killable() and return
    -EINTR if mmap lock acquisition is interrupted or a fatal signal is
    pending during retry handling.
  - Removed the redundant non-timeout hmm_range_fault_unlocked() interface.
  - Updated Documentation/mm/hmm.rst and kernel-doc to describe the timeout API
    and the intended caller pattern.
  - Updated the HMM selftests to use hmm_range_fault_unlocked_timeout()
    only, including coverage for the finite-timeout path.
  - Added in-tree users of the new helper:
      - mshv
      - nouveau
      - RDMA/umem
      - amdxdna
      - drm/gpusvm
  - Preserved each converted driver’s existing timeout convention:
      - unbounded retry where the old code retried indefinitely,
      - HMM_RANGE_DEFAULT_TIMEOUT where the old code used that budget,
      - existing driver-specific timeout return values such as -ETIME.
  - Used max_t(long, timeout - jiffies, 1) when passing remaining time from
    absolute jiffies deadlines to avoid unsigned underflow while keeping a
    minimum one-jiffy retry window.
  - Left callers on hmm_range_fault() when they already need to hold
    mmap_lock across surrounding work, such as drm_gpusvm_check_pages().

Changes in v6:
  - Reworked the new API from the external int *locked pattern to
    hmm_range_fault_unlocked(), which owns mmap_read_lock() internally.
  - Changed the dropped-lock contract: hmm_range_fault_unlocked() now returns
    -EBUSY when the mmap lock is dropped, and callers restart with a fresh
    mmu_interval_read_begin() sequence.
  - Kept hmm_range_fault() as the locked variant for existing users, preserving
    its caller-held mmap lock contract.
  - Added an in-tree user by converting the MSHV region fault path to
    hmm_range_fault_unlocked().
  - Updated Documentation/mm/hmm.rst and kernel-doc to describe the unlocked
    helper and retry pattern.
  - Updated commit messages to match the new API and return semantics.
  - Kept the userfaultfd HMM selftest using the test_hmm unlocked read ioctl
    path.

Changes in v5:
 - Rework hmm_range_fault_unlockable() retry handling to retry
   VM_FAULT_RETRY internally with FAULT_FLAG_TRIED set, matching the
   fixup_user_fault() pattern and avoiding repeated first-retry lock drops.
 - Distinguish VM_FAULT_RETRY from VM_FAULT_COMPLETED: retry faults now
   reacquire the mmap lock internally, while completed faults return to the
   caller with *locked = 0 so the caller can restart with a fresh notifier
   sequence.
 - Document the two *locked return states, including the -EINTR case when a
   fatal signal is pending after the mmap lock has already been dropped.
 - Update comments around HMM_FAULT_UNLOCKED and the HMM fault loop to match
   the current hmm_range_fault_unlockable() implementation.

Changes in v4:
 - Rebased on 7.2-rc1

Changes in v3:
 - Return -EFAULT from dmirror_fault_unlockable() when the mirrored mm can
   no longer be pinned.
 - Add an eventfd stop signal for the userfaultfd handler thread to avoid
   waiting for the poll timeout on successful test completion.


Changes in v2:

 - Split into a preparatory refactor (new patch 1) that moves
   handle_mm_fault() out of the walk callbacks, plus a smaller feature
   patch on top.  Suggested by David Hildenbrand.
 - Hugetlb regions are now supported on the unlockable path; the v1
   -EFAULT short-circuit and the hugetlb_vma_lock_read drop/retake
   dance are gone.
 - Distinct internal sentinels for "needs fault" (HMM_FAULT_PENDING)
   and "lock dropped" (HMM_FAULT_UNLOCKED).
 - Outer loop now re-walks after a successful internal fault so the
   faulted pfns end up in range->hmm_pfns.
 - Kernel-doc on hmm_range_fault_unlockable() and the
   Documentation/mm/hmm.rst example match the implementation.
 - Dropped the mshv driver conversion (v1 patch 2); will post
   separately.
 - Selftest converted to drive the path through test_hmm with a
   userfaultfd handler (new HMM_DMIRROR_READ_UNLOCKABLE ioctl).

---

Stanislav Kinsburskii (8):
      mm/hmm: move page fault handling out of walk callbacks
      mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support
      selftests/mm: add HMM tests for mmap lock-dropping faults
      mshv: Use hmm_range_fault_unlocked_timeout() for region faults
      drm/nouveau: Use hmm_range_fault_unlocked_timeout() for SVM faults
      RDMA/umem: Use hmm_range_fault_unlocked_timeout() for ODP faults
      accel/amdxdna: Use hmm_range_fault_unlocked_timeout() for range population
      drm/gpusvm: Use hmm_range_fault_unlocked_timeout() for range faults


 Documentation/mm/hmm.rst               |   63 ++++++++
 drivers/accel/amdxdna/aie2_ctx.c       |   17 --
 drivers/gpu/drm/drm_gpusvm.c           |   52 +------
 drivers/gpu/drm/nouveau/nouveau_svm.c  |   11 -
 drivers/hv/mshv_regions.c              |   54 +------
 drivers/infiniband/core/umem_odp.c     |   18 +-
 include/linux/hmm.h                    |    2 
 lib/test_hmm.c                         |  111 ++++++++++++++
 lib/test_hmm_uapi.h                    |    3 
 mm/hmm.c                               |  246 ++++++++++++++++++++++++--------
 tools/testing/selftests/mm/hmm-tests.c |  220 +++++++++++++++++++++++++++++
 11 files changed, 613 insertions(+), 184 deletions(-)

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