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 entry point,
hmm_range_fault_unlockable(), 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.
Faulting hugetlb pages on the unlockable path is not supported because
walk_hugetlb_range() unconditionally holds and releases
hugetlb_vma_lock_read across the callback; if the mmap lock is dropped
inside the callback, the VMA may be freed before the walk framework's
unlock. Hugetlb pages already present in page tables are handled normally.
Possible approaches to lift this limitation are documented in
Documentation/mm/hmm.rst.
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 (3):
mm/hmm: move page fault handling out of walk callbacks
mm/hmm: add hmm_range_fault_unlockable() for mmap lock-drop support
selftests/mm: add userfaultfd test for HMM unlockable path
Documentation/mm/hmm.rst | 62 +++++++++++
include/linux/hmm.h | 1
lib/test_hmm.c | 122 +++++++++++++++++++++
lib/test_hmm_uapi.h | 1
mm/hmm.c | 187 ++++++++++++++++++++++++--------
tools/testing/selftests/mm/hmm-tests.c | 132 +++++++++++++++++++++++
6 files changed, 461 insertions(+), 44 deletions(-)
