[PATCH v10 0/8] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

[Stanislav Kinsburskii <skinsburskii@gmail.com>]

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 v10:
  - Included contended mmap_lock acquisition in the
    hmm_range_fault_unlocked_timeout() retry budget.
  - Dropped the redundant top-level fatal_signal_pending() check in the HMM
    unlocked retry loop; mmap_read_lock_killable() now covers that path.
  - Restored the absolute outer timeout in drm_gpusvm_get_pages(), since it can
    run from GPU page-fault workers and must not rely on the worker task’s fatal
    signal state to stop invalidation retries.
 
Changes in v9:

  - Folded the fixups into the full 8-patch series instead of sending a
    separate fixup series.
  - Clarified that the HMM timeout bounds repeated HMM/mmu-notifier retry
    attempts, with the helper refreshing range->notifier_seq internally.
  - Kept nouveau’s explicit outer deadline because its retry loop runs in a GPU
    fault worker and cannot rely on fatal signals from the faulting process.
  - Converted amdxdna, and GPU SVM callers to pass the shole timeout budget to
    hmm_range_fault_unlocked_timeout().

Changes in v8:

  - Make hmm_range_fault_unlocked_timeout() the primary documented HMM
    range-fault API, and move hmm_range_fault() into the “use only if the
    caller really must hold mmap_lock” category.
  - Clarify that the timeout is a retry budget for repeated mmu-notifier
    invalidation retries. HMM does not interrupt an in-progress page fault
    when the timeout expires.
  - Restart the retry timeout only when handle_mm_fault() dropped
    mmap_lock, because that indicates a lock-dropping fault handler such as
    userfaultfd made progress. Ordinary -EBUSY retries keep the existing
    deadline.
  - Remove the attempted timeout selftest. The remaining selftest covers the
    intended userfaultfd path by resolving missing-page faults through
    HMM_DMIRROR_READ_UNLOCKED and hmm_range_fault_unlocked_timeout(..., 0).

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 test 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               |  79 +++++++---
 drivers/accel/amdxdna/aie2_ctx.c       |  23 +--
 drivers/gpu/drm/drm_gpusvm.c           |  60 ++------
 drivers/gpu/drm/nouveau/nouveau_svm.c  |  20 +--
 drivers/hv/mshv_regions.c              |  54 ++-----
 drivers/infiniband/core/umem_odp.c     |  18 +--
 include/linux/hmm.h                    |   2 +
 lib/test_hmm.c                         | 107 +++++++++++++-
 lib/test_hmm_uapi.h                    |   1 +
 mm/hmm.c                               | 256 +++++++++++++++++++++++++--------
 tools/testing/selftests/mm/hmm-tests.c | 150 +++++++++++++++++++
 11 files changed, 551 insertions(+), 219 deletions(-)
---
base-commit: a4271215228d2bd2a50eea03ab0a948b36036948
change-id: 20260722-hmm-v10-727db2bb9d34

Best regards,
-- 
Stanislav Kinsburskii <skinsburskii@gmail.com>