mm/hmm: add hmm_range_fault_unlocked_timeout() for mmap lock-drop support

hmm_range_fault() requires the caller to hold the mmap read lock for the
duration of the call.  This is incompatible with mappings whose fault
handler may release the mmap lock, notably userfaultfd-managed regions,
where handle_mm_fault() can return VM_FAULT_RETRY or VM_FAULT_COMPLETED
after dropping the lock.  Drivers that need to populate device page tables
for such mappings have no way to do so today.

Add hmm_range_fault_unlocked_timeout() for callers that do not need to
hold mmap_lock across any work outside the HMM fault itself.  The helper
takes mmap_read_lock_killable() internally, calls the common HMM fault
implementation, and releases the lock before returning if it is still
held.  The timeout is specified in jiffies; passing 0 retries
indefinitely, while a non-zero timeout makes the helper return -EBUSY when
the retry budget expires.  The retry deadline is set before refreshing the
notifier sequence and acquiring mmap_lock, so contended mmap_lock
acquisition is included in the retry budget.  After acquiring mmap_lock,
the helper also rejects unstable address spaces before walking page
tables.

When handle_mm_fault() drops mmap_lock, or when the range is invalidated,
hmm_range_fault_unlocked_timeout() refreshes range->notifier_seq and
retries the walk internally.  If the lock was dropped, the retry deadline
is also restarted because a lock-dropping fault handler made progress. 
Ordinary -EBUSY retries keep the existing deadline, preserving the
caller's timeout policy for repeated mmu-notifier invalidations.

The caller only needs to perform the usual post-success
mmu_interval_read_retry() check while holding its update lock before
consuming the pfns.  If mmap_lock acquisition is interrupted or a fatal
signal is pending during retry handling, -EINTR is returned instead.

The common implementation conditionally sets FAULT_FLAG_ALLOW_RETRY and
FAULT_FLAG_KILLABLE only for hmm_range_fault_unlocked_timeout().  The
existing hmm_range_fault() path still passes no locked state, does not
allow handle_mm_fault() to drop mmap_lock, and remains a thin wrapper
preserving the existing API contract for current callers.

The previous refactor that moved page fault handling out of the page-table
walk callbacks is what makes this change small.  Faults now run after
walk_page_range() has unwound, with only mmap_lock held, so dropping it
does not interact with the walker's pte spinlock or hugetlb_vma_lock. 
Hugetlb regions therefore participate in the unlocked path uniformly with
PTE- and PMD-level mappings; no special case is required.

Documentation/mm/hmm.rst is updated with a description of the new API and
the recommended caller pattern.

Link: https://lore.kernel.org/20260723-hmm-v10-v11-2-c55b003a4b61@gmail.com
Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Dave Airlie <airlied@gmail.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lizhi Hou <lizhi.hou@amd.com>
Cc: Long Li <longli@microsoft.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Lyude <lyude@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oded Gabbay <ogabbay@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Zimemrmann <tzimmermann@suse.de>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions