[PATCH v6 0/4] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

[PATCH v6 0/4] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings

[Stanislav Kinsburskii]

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 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 (4):
      mm/hmm: move page fault handling out of walk callbacks
      mm/hmm: add hmm_range_fault_unlocked() for mmap lock-drop support
      selftests/mm: add userfaultfd test for HMM unlocked path
      mshv: Use hmm_range_fault_unlocked() for region faults


 Documentation/mm/hmm.rst               |   59 ++++++++
 drivers/hv/mshv_regions.c              |   14 +-
 include/linux/hmm.h                    |    1 
 lib/test_hmm.c                         |  114 ++++++++++++++++
 lib/test_hmm_uapi.h                    |    1 
 mm/hmm.c                               |  223 +++++++++++++++++++++++---------
 tools/testing/selftests/mm/hmm-tests.c |  149 +++++++++++++++++++++
 7 files changed, 492 insertions(+), 69 deletions(-)

[PATCH v6 1/4] mm/hmm: move page fault handling out of walk callbacks

[Stanislav Kinsburskii]

hmm_range_fault() currently triggers page faults from inside the page-table
walk callbacks: hmm_vma_walk_pmd(), hmm_vma_walk_pud(),
hmm_vma_walk_hugetlb_entry() and the pte-level helper all call
hmm_vma_fault(), which in turn calls handle_mm_fault() while the walker
still holds nested locks.  The pte spinlock is dropped explicitly by each
caller, and the hugetlb path manually drops and retakes
hugetlb_vma_lock_read around the fault to dodge a deadlock against the walk
framework's unconditional unlock.

This layering does not extend cleanly to fault handlers that may release
mmap_lock (VM_FAULT_RETRY, VM_FAULT_COMPLETED). If the lock is dropped
while walk_page_range() is mid-traversal, the VMA can be freed before the
walk framework's matching hugetlb_vma_unlock_read(), turning that unlock
into a use-after-free.

Split the responsibilities the way get_user_pages() does. Walk callbacks
become inspect-only: when they detect a range that needs to be faulted in,
they record it in struct hmm_vma_walk and return a private sentinel
(HMM_FAULT_PENDING). The outer loop in hmm_range_fault() then drops out of
walk_page_range(), invokes a new helper hmm_do_fault() that calls
handle_mm_fault() with only mmap_lock held, and restarts the walk so the
now-present entries are collected into hmm_pfns.

No functional change for existing callers. As a side effect the hugetlb
callback no longer needs the hugetlb_vma_{un}lock_read dance, and every
fault-path exit from the callbacks now releases the pte spinlock on a
single, common path. This refactor is also a precursor for adding an
unlockable variant of hmm_range_fault() in a follow-up patch.

Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
---
 mm/hmm.c |  118 +++++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 75 insertions(+), 43 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index 4f3f627d2b47..2129b1ee4c35 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -33,8 +33,17 @@
 struct hmm_vma_walk {
 	struct hmm_range	*range;
 	unsigned long		last;
+	unsigned long		end;
+	unsigned int		required_fault;
 };
 
+/*
+ * Internal sentinel returned by walk callbacks when they need a page fault.
+ * The callback stores end/required_fault in hmm_vma_walk; the outer loop
+ * consumes the sentinel and never propagates it to the caller.
+ */
+#define HMM_FAULT_PENDING	-EAGAIN
+
 enum {
 	HMM_NEED_FAULT = 1 << 0,
 	HMM_NEED_WRITE_FAULT = 1 << 1,
@@ -60,37 +69,25 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end,
 }
 
 /*
- * hmm_vma_fault() - fault in a range lacking valid pmd or pte(s)
- * @addr: range virtual start address (inclusive)
- * @end: range virtual end address (exclusive)
- * @required_fault: HMM_NEED_* flags
- * @walk: mm_walk structure
- * Return: -EBUSY after page fault, or page fault error
+ * hmm_record_fault() - record a range that needs to be faulted in
  *
- * This function will be called whenever pmd_none() or pte_none() returns true,
- * or whenever there is no page directory covering the virtual address range.
+ * Called by the walk callbacks when they discover that part of the range
+ * needs a page fault.  The callback records what to fault and returns
+ * HMM_FAULT_PENDING; the outer loop in hmm_range_fault() drops back out of
+ * walk_page_range() and invokes handle_mm_fault() from a context where no
+ * page-table or hugetlb_vma_lock is held.
  */
-static int hmm_vma_fault(unsigned long addr, unsigned long end,
-			 unsigned int required_fault, struct mm_walk *walk)
+static int hmm_record_fault(unsigned long addr, unsigned long end,
+			    unsigned int required_fault,
+			    struct mm_walk *walk)
 {
 	struct hmm_vma_walk *hmm_vma_walk = walk->private;
-	struct vm_area_struct *vma = walk->vma;
-	unsigned int fault_flags = FAULT_FLAG_REMOTE;
 
 	WARN_ON_ONCE(!required_fault);
 	hmm_vma_walk->last = addr;
-
-	if (required_fault & HMM_NEED_WRITE_FAULT) {
-		if (!(vma->vm_flags & VM_WRITE))
-			return -EPERM;
-		fault_flags |= FAULT_FLAG_WRITE;
-	}
-
-	for (; addr < end; addr += PAGE_SIZE)
-		if (handle_mm_fault(vma, addr, fault_flags, NULL) &
-		    VM_FAULT_ERROR)
-			return -EFAULT;
-	return -EBUSY;
+	hmm_vma_walk->end = end;
+	hmm_vma_walk->required_fault = required_fault;
+	return HMM_FAULT_PENDING;
 }
 
 static unsigned int hmm_pte_need_fault(const struct hmm_vma_walk *hmm_vma_walk,
@@ -174,7 +171,7 @@ static int hmm_vma_walk_hole(unsigned long addr, unsigned long end,
 		return hmm_pfns_fill(addr, end, range, HMM_PFN_ERROR);
 	}
 	if (required_fault)
-		return hmm_vma_fault(addr, end, required_fault, walk);
+		return hmm_record_fault(addr, end, required_fault, walk);
 	return hmm_pfns_fill(addr, end, range, 0);
 }
 
@@ -209,7 +206,7 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
 	required_fault =
 		hmm_range_need_fault(hmm_vma_walk, hmm_pfns, npages, cpu_flags);
 	if (required_fault)
-		return hmm_vma_fault(addr, end, required_fault, walk);
+		return hmm_record_fault(addr, end, required_fault, walk);
 
 	pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
 	for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
@@ -328,7 +325,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 fault:
 	pte_unmap(ptep);
 	/* Fault any virtual address we were asked to fault */
-	return hmm_vma_fault(addr, end, required_fault, walk);
+	return hmm_record_fault(addr, end, required_fault, walk);
 }
 
 #ifdef CONFIG_ARCH_SUPPORTS_PMD_SOFTLEAF
@@ -371,7 +368,7 @@ static int hmm_vma_handle_absent_pmd(struct mm_walk *walk, unsigned long start,
 					      npages, 0);
 	if (required_fault) {
 		if (softleaf_is_device_private(entry))
-			return hmm_vma_fault(addr, end, required_fault, walk);
+			return hmm_record_fault(addr, end, required_fault, walk);
 		else
 			return -EFAULT;
 	}
@@ -517,7 +514,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 						      npages, cpu_flags);
 		if (required_fault) {
 			spin_unlock(ptl);
-			return hmm_vma_fault(addr, end, required_fault, walk);
+			return hmm_record_fault(addr, end, required_fault, walk);
 		}
 
 		pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
@@ -564,21 +561,8 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask,
 	required_fault =
 		hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags);
 	if (required_fault) {
-		int ret;
-
 		spin_unlock(ptl);
-		hugetlb_vma_unlock_read(vma);
-		/*
-		 * Avoid deadlock: drop the vma lock before calling
-		 * hmm_vma_fault(), which will itself potentially take and
-		 * drop the vma lock. This is also correct from a
-		 * protection point of view, because there is no further
-		 * use here of either pte or ptl after dropping the vma
-		 * lock.
-		 */
-		ret = hmm_vma_fault(addr, end, required_fault, walk);
-		hugetlb_vma_lock_read(vma);
-		return ret;
+		return hmm_record_fault(addr, end, required_fault, walk);
 	}
 
 	pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT);
@@ -637,6 +621,44 @@ static const struct mm_walk_ops hmm_walk_ops = {
 	.walk_lock	= PGWALK_RDLOCK,
 };
 
+/*
+ * hmm_do_fault - fault in a range recorded by a walk callback
+ *
+ * Called from the outer loop in hmm_range_fault() after a callback
+ * returned HMM_FAULT_PENDING.  At this point we hold only mmap_lock;
+ * the page-table spinlock and any hugetlb_vma_lock acquired by the walk
+ * framework have already been released by the unwind.
+ *
+ * Returns -EBUSY on success (all pages faulted, caller should re-walk).
+ * Returns a negative errno on failure.
+ */
+static int hmm_do_fault(struct mm_struct *mm,
+			struct hmm_vma_walk *hmm_vma_walk)
+{
+	unsigned long addr = hmm_vma_walk->last;
+	unsigned long end = hmm_vma_walk->end;
+	unsigned int required_fault = hmm_vma_walk->required_fault;
+	unsigned int fault_flags = FAULT_FLAG_REMOTE;
+	struct vm_area_struct *vma;
+
+	vma = vma_lookup(mm, addr);
+	if (!vma)
+		return -EFAULT;
+
+	if (required_fault & HMM_NEED_WRITE_FAULT) {
+		if (!(vma->vm_flags & VM_WRITE))
+			return -EPERM;
+		fault_flags |= FAULT_FLAG_WRITE;
+	}
+
+	for (; addr < end; addr += PAGE_SIZE)
+		if (handle_mm_fault(vma, addr, fault_flags, NULL) &
+		    VM_FAULT_ERROR)
+			return -EFAULT;
+
+	return -EBUSY;
+}
+
 /**
  * hmm_range_fault - try to fault some address in a virtual address range
  * @range:	argument structure
@@ -674,6 +696,16 @@ int hmm_range_fault(struct hmm_range *range)
 			return -EBUSY;
 		ret = walk_page_range(mm, hmm_vma_walk.last, range->end,
 				      &hmm_walk_ops, &hmm_vma_walk);
+		/*
+		 * When HMM_FAULT_PENDING is returned a walk callback
+		 * recorded a range that needs handle_mm_fault();
+		 * hmm_do_fault() runs the fault outside walk_page_range()
+		 * (so no page-table or hugetlb_vma_lock is held) and
+		 * returns -EBUSY so the loop re-walks and picks up the
+		 * now-present entries.
+		 */
+		if (ret == HMM_FAULT_PENDING)
+			ret = hmm_do_fault(mm, &hmm_vma_walk);
 		/*
 		 * When -EBUSY is returned the loop restarts with
 		 * hmm_vma_walk.last set to an address that has not been stored

[PATCH v6 2/4] mm/hmm: add hmm_range_fault_unlocked() for mmap lock-drop support

[Stanislav Kinsburskii]

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() for callers that do not need to hold
mmap_lock across any work outside the HMM fault itself. The new helper
takes mmap_read_lock() internally, calls the common HMM fault
implementation, and releases the lock before returning if it is still
held.

When handle_mm_fault() drops mmap_lock, hmm_range_fault_unlocked() returns
-EBUSY. The caller must then restart its HMM walk with a fresh
mmu_interval_read_begin() sequence. If a fatal signal is pending after the
lock has been dropped, -EINTR is returned instead. This gives callers a
simple restart contract while allowing fault handlers that must drop
mmap_lock, including userfaultfd-backed mappings, to make progress.

The common implementation conditionally sets FAULT_FLAG_ALLOW_RETRY and
FAULT_FLAG_KILLABLE only for hmm_range_fault_unlocked(). 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.

Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
---
 Documentation/mm/hmm.rst |   59 +++++++++++++++++++++
 include/linux/hmm.h      |    1 
 mm/hmm.c                 |  129 ++++++++++++++++++++++++++++++++++++----------
 3 files changed, 160 insertions(+), 29 deletions(-)

diff --git a/Documentation/mm/hmm.rst b/Documentation/mm/hmm.rst
index 7d61b7a8b65b..844343712abd 100644
--- a/Documentation/mm/hmm.rst
+++ b/Documentation/mm/hmm.rst
@@ -208,6 +208,65 @@ invalidate() callback. That lock must be held before calling
 mmu_interval_read_retry() to avoid any race with a concurrent CPU page table
 update.
 
+Dropping the mmap lock during page faults
+=========================================
+
+Some VMAs have fault handlers that need to release the mmap lock while
+servicing a fault (for example, regions managed by ``userfaultfd``).
+``hmm_range_fault()`` cannot be used on such mappings because it must hold the
+mmap lock for the duration of the call. Drivers that need to support them
+should call::
+
+  int hmm_range_fault_unlocked(struct hmm_range *range);
+
+The caller must not hold ``mmap_read_lock`` before the call.
+``hmm_range_fault_unlocked()`` takes the mmap read lock internally and allows
+``handle_mm_fault()`` to drop it during fault handling. If the mmap lock is
+dropped, the function returns ``-EBUSY``. The caller must then restart the
+walk from ``range->start`` with a fresh notifier sequence. ``-EINTR`` is
+returned if a fatal signal is pending during retry handling.
+
+A typical caller looks like this::
+
+ int driver_populate_range_unlocked(...)
+ {
+      struct hmm_range range;
+      ...
+
+      range.notifier = &interval_sub;
+      range.start = ...;
+      range.end = ...;
+      range.hmm_pfns = ...;
+
+      if (!mmget_not_zero(interval_sub.mm))
+          return -EFAULT;
+
+ again:
+      range.notifier_seq = mmu_interval_read_begin(&interval_sub);
+      ret = hmm_range_fault_unlocked(&range);
+      if (ret) {
+          if (ret == -EBUSY)
+              goto again;
+          goto out_put;
+      }
+
+      take_lock(driver->update);
+      if (mmu_interval_read_retry(&interval_sub, range.notifier_seq)) {
+          release_lock(driver->update);
+          goto again;
+      }
+
+      /* Use pfns array content to update device page table,
+       * under the update lock */
+
+      release_lock(driver->update);
+      ret = 0;
+
+ out_put:
+      mmput(interval_sub.mm);
+      return ret;
+ }
+
 Leverage default_flags and pfn_flags_mask
 =========================================
 
diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index db75ffc949a7..1b584e5a2900 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -123,6 +123,7 @@ struct hmm_range {
  * Please see Documentation/mm/hmm.rst for how to use the range API.
  */
 int hmm_range_fault(struct hmm_range *range);
+int hmm_range_fault_unlocked(struct hmm_range *range);
 
 /*
  * HMM_RANGE_DEFAULT_TIMEOUT - default timeout (ms) when waiting for a range
diff --git a/mm/hmm.c b/mm/hmm.c
index 2129b1ee4c35..316116994209 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -32,6 +32,7 @@
 
 struct hmm_vma_walk {
 	struct hmm_range	*range;
+	int			*locked;
 	unsigned long		last;
 	unsigned long		end;
 	unsigned int		required_fault;
@@ -44,6 +45,14 @@ struct hmm_vma_walk {
  */
 #define HMM_FAULT_PENDING	-EAGAIN
 
+/*
+ * Internal sentinel returned by hmm_do_fault() when handle_mm_fault()
+ * completes a page fault with the mmap lock dropped. hmm_do_fault() sets
+ * *locked = 0; the outer loop consumes the sentinel and never propagates it
+ * to the caller.
+ */
+#define HMM_FAULT_UNLOCKED	-ENOLCK
+
 enum {
 	HMM_NEED_FAULT = 1 << 0,
 	HMM_NEED_WRITE_FAULT = 1 << 1,
@@ -73,9 +82,9 @@ static int hmm_pfns_fill(unsigned long addr, unsigned long end,
  *
  * Called by the walk callbacks when they discover that part of the range
  * needs a page fault.  The callback records what to fault and returns
- * HMM_FAULT_PENDING; the outer loop in hmm_range_fault() drops back out of
- * walk_page_range() and invokes handle_mm_fault() from a context where no
- * page-table or hugetlb_vma_lock is held.
+ * HMM_FAULT_PENDING; the outer loop in hmm_range_fault_locked() drops
+ * back out of walk_page_range() and invokes handle_mm_fault() from a context
+ * where no page-table or hugetlb_vma_lock is held.
  */
 static int hmm_record_fault(unsigned long addr, unsigned long end,
 			    unsigned int required_fault,
@@ -624,7 +633,7 @@ static const struct mm_walk_ops hmm_walk_ops = {
 /*
  * hmm_do_fault - fault in a range recorded by a walk callback
  *
- * Called from the outer loop in hmm_range_fault() after a callback
+ * Called from the outer loop in hmm_range_fault_locked() after a callback
  * returned HMM_FAULT_PENDING.  At this point we hold only mmap_lock;
  * the page-table spinlock and any hugetlb_vma_lock acquired by the walk
  * framework have already been released by the unwind.
@@ -641,6 +650,9 @@ static int hmm_do_fault(struct mm_struct *mm,
 	unsigned int fault_flags = FAULT_FLAG_REMOTE;
 	struct vm_area_struct *vma;
 
+	if (hmm_vma_walk->locked)
+		fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
+
 	vma = vma_lookup(mm, addr);
 	if (!vma)
 		return -EFAULT;
@@ -651,37 +663,33 @@ static int hmm_do_fault(struct mm_struct *mm,
 		fault_flags |= FAULT_FLAG_WRITE;
 	}
 
-	for (; addr < end; addr += PAGE_SIZE)
-		if (handle_mm_fault(vma, addr, fault_flags, NULL) &
-		    VM_FAULT_ERROR)
-			return -EFAULT;
+	for (; addr < end; addr += PAGE_SIZE) {
+		vm_fault_t ret;
+
+		ret = handle_mm_fault(vma, addr, fault_flags, NULL);
+
+		if (ret & (VM_FAULT_COMPLETED | VM_FAULT_RETRY)) {
+			*hmm_vma_walk->locked = 0;
+			return HMM_FAULT_UNLOCKED;
+		}
+
+		if (ret & VM_FAULT_ERROR) {
+			int err = vm_fault_to_errno(ret, 0);
+
+			if (err)
+				return err;
+			BUG();
+		}
+	}
 
 	return -EBUSY;
 }
 
-/**
- * hmm_range_fault - try to fault some address in a virtual address range
- * @range:	argument structure
- *
- * Returns 0 on success or one of the following error codes:
- *
- * -EINVAL:	Invalid arguments or mm or virtual address is in an invalid vma
- *		(e.g., device file vma).
- * -ENOMEM:	Out of memory.
- * -EPERM:	Invalid permission (e.g., asking for write and range is read
- *		only).
- * -EBUSY:	The range has been invalidated and the caller needs to wait for
- *		the invalidation to finish.
- * -EFAULT:     A page was requested to be valid and could not be made valid
- *              ie it has no backing VMA or it is illegal to access
- *
- * This is similar to get_user_pages(), except that it can read the page tables
- * without mutating them (ie causing faults).
- */
-int hmm_range_fault(struct hmm_range *range)
+static int hmm_range_fault_locked(struct hmm_range *range, int *locked)
 {
 	struct hmm_vma_walk hmm_vma_walk = {
 		.range = range,
+		.locked = locked,
 		.last = range->start,
 	};
 	struct mm_struct *mm = range->notifier->mm;
@@ -704,8 +712,14 @@ int hmm_range_fault(struct hmm_range *range)
 		 * returns -EBUSY so the loop re-walks and picks up the
 		 * now-present entries.
 		 */
-		if (ret == HMM_FAULT_PENDING)
+		if (ret == HMM_FAULT_PENDING) {
 			ret = hmm_do_fault(mm, &hmm_vma_walk);
+			if (ret == HMM_FAULT_UNLOCKED) {
+				if (fatal_signal_pending(current))
+					return -EINTR;
+				return -EBUSY;
+			}
+		}
 		/*
 		 * When -EBUSY is returned the loop restarts with
 		 * hmm_vma_walk.last set to an address that has not been stored
@@ -715,8 +729,65 @@ int hmm_range_fault(struct hmm_range *range)
 	} while (ret == -EBUSY);
 	return ret;
 }
+
+/**
+ * hmm_range_fault - try to fault some address in a virtual address range
+ * @range:	argument structure
+ *
+ * Returns 0 on success or one of the following error codes:
+ *
+ * -EINVAL:	Invalid arguments or mm or virtual address is in an invalid vma
+ *		(e.g., device file vma).
+ * -ENOMEM:	Out of memory.
+ * -EPERM:	Invalid permission (e.g., asking for write and range is read
+ *		only).
+ * -EBUSY:	The range has been invalidated and the caller needs to wait for
+ *		the invalidation to finish.
+ * -EFAULT:     A page was requested to be valid and could not be made valid
+ *              ie it has no backing VMA or it is illegal to access
+ *
+ * This is similar to get_user_pages(), except that it can read the page tables
+ * without mutating them (ie causing faults).
+ *
+ * The mmap lock must be held by the caller and will remain held on return.
+ * For a variant that allows the mmap lock to be dropped during faults (e.g.,
+ * for userfaultfd support), see hmm_range_fault_unlocked().
+ */
+int hmm_range_fault(struct hmm_range *range)
+{
+	return hmm_range_fault_locked(range, NULL);
+}
 EXPORT_SYMBOL(hmm_range_fault);
 
+/**
+ * hmm_range_fault_unlocked - fault in a range, possibly dropping the mmap lock
+ * @range:	argument structure
+ *
+ * This is similar to hmm_range_fault(), except the caller must not hold the
+ * mmap lock. The function takes the mmap read lock internally and allows
+ * handle_mm_fault() to drop it during faults. If the lock is dropped, the
+ * function returns -EBUSY and the caller must restart the walk with a fresh
+ * mmu_interval_read_begin() sequence.
+ *
+ * Returns 0 on success or one of the error codes documented for
+ * hmm_range_fault(). -EINTR is returned if a fatal signal is pending during
+ * retry handling.
+ */
+int hmm_range_fault_unlocked(struct hmm_range *range)
+{
+	struct mm_struct *mm = range->notifier->mm;
+	int locked = 1;
+	int ret;
+
+	mmap_read_lock(mm);
+	ret = hmm_range_fault_locked(range, &locked);
+	if (locked)
+		mmap_read_unlock(mm);
+
+	return ret;
+}
+EXPORT_SYMBOL(hmm_range_fault_unlocked);
+
 /**
  * hmm_dma_map_alloc - Allocate HMM map structure
  * @dev: device to allocate structure for

[PATCH v6 3/4] selftests/mm: add userfaultfd test for HMM unlocked path

[Stanislav Kinsburskii]

Add a selftest that exercises hmm_range_fault_unlocked() with a
userfaultfd-backed mapping. The test:

1. Creates an anonymous mmap region
2. Registers it with userfaultfd (UFFDIO_REGISTER_MODE_MISSING)
3. Spawns a handler thread that responds to page faults by filling
   pages with a known pattern (0xAB) via UFFDIO_COPY
4. Issues HMM_DMIRROR_READ_UNLOCKED to the test_hmm driver, which
   calls hmm_range_fault_unlocked() internally
5. Verifies the device read back the data provided by the userfaultfd
   handler

This requires changes to the test_hmm kernel module:
- New dmirror_range_fault_unlocked() that uses the new HMM API
- New dmirror_fault_unlocked() and dmirror_read_unlocked() wrappers
- New HMM_DMIRROR_READ_UNLOCKED ioctl (0x09)

Assisted-by: GitHub-Copilot:claude-opus-4.6
Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
---
 lib/test_hmm.c                         |  114 ++++++++++++++++++++++++
 lib/test_hmm_uapi.h                    |    1 
 tools/testing/selftests/mm/hmm-tests.c |  149 ++++++++++++++++++++++++++++++++
 3 files changed, 264 insertions(+)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 45c0cb992218..dcc3a23bb983 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -389,6 +389,76 @@ static int dmirror_range_fault(struct dmirror *dmirror,
 	return ret;
 }
 
+static int dmirror_range_fault_unlocked(struct dmirror *dmirror,
+					struct hmm_range *range)
+{
+	unsigned long timeout =
+		jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
+	int ret;
+
+	while (true) {
+		if (time_after(jiffies, timeout)) {
+			ret = -EBUSY;
+			goto out;
+		}
+
+		range->notifier_seq = mmu_interval_read_begin(range->notifier);
+		ret = hmm_range_fault_unlocked(range);
+		if (ret) {
+			if (ret == -EBUSY)
+				continue;
+			goto out;
+		}
+
+		mutex_lock(&dmirror->mutex);
+		if (mmu_interval_read_retry(range->notifier,
+					    range->notifier_seq)) {
+			mutex_unlock(&dmirror->mutex);
+			continue;
+		}
+		break;
+	}
+
+	ret = dmirror_do_fault(dmirror, range);
+
+	mutex_unlock(&dmirror->mutex);
+out:
+	return ret;
+}
+
+static int dmirror_fault_unlocked(struct dmirror *dmirror,
+				  unsigned long start,
+				  unsigned long end, bool write)
+{
+	struct mm_struct *mm = dmirror->notifier.mm;
+	unsigned long addr;
+	unsigned long pfns[32];
+	struct hmm_range range = {
+		.notifier = &dmirror->notifier,
+		.hmm_pfns = pfns,
+		.pfn_flags_mask = 0,
+		.default_flags =
+			HMM_PFN_REQ_FAULT | (write ? HMM_PFN_REQ_WRITE : 0),
+		.dev_private_owner = dmirror->mdevice,
+	};
+	int ret = 0;
+
+	if (!mmget_not_zero(mm))
+		return -EFAULT;
+
+	for (addr = start; addr < end; addr = range.end) {
+		range.start = addr;
+		range.end = min(addr + (ARRAY_SIZE(pfns) << PAGE_SHIFT), end);
+
+		ret = dmirror_range_fault_unlocked(dmirror, &range);
+		if (ret)
+			break;
+	}
+
+	mmput(mm);
+	return ret;
+}
+
 static int dmirror_fault(struct dmirror *dmirror, unsigned long start,
 			 unsigned long end, bool write)
 {
@@ -488,6 +558,47 @@ static int dmirror_read(struct dmirror *dmirror, struct hmm_dmirror_cmd *cmd)
 	return ret;
 }
 
+static int dmirror_read_unlocked(struct dmirror *dmirror,
+				 struct hmm_dmirror_cmd *cmd)
+{
+	struct dmirror_bounce bounce;
+	unsigned long start, end;
+	unsigned long size = cmd->npages << PAGE_SHIFT;
+	int ret;
+
+	start = cmd->addr;
+	end = start + size;
+	if (end < start)
+		return -EINVAL;
+
+	ret = dmirror_bounce_init(&bounce, start, size);
+	if (ret)
+		return ret;
+
+	while (1) {
+		mutex_lock(&dmirror->mutex);
+		ret = dmirror_do_read(dmirror, start, end, &bounce);
+		mutex_unlock(&dmirror->mutex);
+		if (ret != -ENOENT)
+			break;
+
+		start = cmd->addr + (bounce.cpages << PAGE_SHIFT);
+		ret = dmirror_fault_unlocked(dmirror, start, end, false);
+		if (ret)
+			break;
+		cmd->faults++;
+	}
+
+	if (ret == 0) {
+		if (copy_to_user(u64_to_user_ptr(cmd->ptr), bounce.ptr,
+				 bounce.size))
+			ret = -EFAULT;
+	}
+	cmd->cpages = bounce.cpages;
+	dmirror_bounce_fini(&bounce);
+	return ret;
+}
+
 static int dmirror_do_write(struct dmirror *dmirror, unsigned long start,
 			    unsigned long end, struct dmirror_bounce *bounce)
 {
@@ -1572,6 +1683,9 @@ static long dmirror_fops_unlocked_ioctl(struct file *filp,
 		dmirror->flags = cmd.npages;
 		ret = 0;
 		break;
+	case HMM_DMIRROR_READ_UNLOCKED:
+		ret = dmirror_read_unlocked(dmirror, &cmd);
+		break;
 
 	default:
 		return -EINVAL;
diff --git a/lib/test_hmm_uapi.h b/lib/test_hmm_uapi.h
index f94c6d457338..ea9b0ec404fb 100644
--- a/lib/test_hmm_uapi.h
+++ b/lib/test_hmm_uapi.h
@@ -38,6 +38,7 @@ struct hmm_dmirror_cmd {
 #define HMM_DMIRROR_CHECK_EXCLUSIVE	_IOWR('H', 0x06, struct hmm_dmirror_cmd)
 #define HMM_DMIRROR_RELEASE		_IOWR('H', 0x07, struct hmm_dmirror_cmd)
 #define HMM_DMIRROR_FLAGS		_IOWR('H', 0x08, struct hmm_dmirror_cmd)
+#define HMM_DMIRROR_READ_UNLOCKED	_IOWR('H', 0x09, struct hmm_dmirror_cmd)
 
 #define HMM_DMIRROR_FLAG_FAIL_ALLOC	(1ULL << 0)
 
diff --git a/tools/testing/selftests/mm/hmm-tests.c b/tools/testing/selftests/mm/hmm-tests.c
index 2f2b9879d100..71765c4d80f3 100644
--- a/tools/testing/selftests/mm/hmm-tests.c
+++ b/tools/testing/selftests/mm/hmm-tests.c
@@ -29,6 +29,10 @@
 #include <sys/mman.h>
 #include <sys/ioctl.h>
 #include <sys/time.h>
+#include <sys/syscall.h>
+#include <sys/eventfd.h>
+#include <linux/userfaultfd.h>
+#include <poll.h>
 
 /*
  * This is a private UAPI to the kernel test module so it isn't exported
@@ -2949,4 +2953,149 @@ TEST_F_TIMEOUT(hmm, benchmark_thp_migration, 120)
 					&thp_results, &regular_results);
 	}
 }
+/*
+ * Test that HMM can fault in pages backed by userfaultfd using the
+ * hmm_range_fault_unlocked() path. This exercises the lock-drop retry
+ * logic in the HMM framework.
+ */
+struct uffd_thread_args {
+	int uffd;
+	int stop_fd;
+	void *page_buffer;
+	unsigned long page_size;
+};
+
+static void *uffd_handler_thread(void *arg)
+{
+	struct uffd_thread_args *args = arg;
+	struct uffd_msg msg;
+	struct uffdio_copy copy;
+	struct pollfd pollfd[2];
+	int ret;
+
+	pollfd[0].fd = args->uffd;
+	pollfd[0].events = POLLIN;
+	pollfd[1].fd = args->stop_fd;
+	pollfd[1].events = POLLIN;
+
+	while (1) {
+		ret = poll(pollfd, 2, -1);
+		if (ret <= 0)
+			break;
+		if (pollfd[1].revents)
+			break;
+		if (!(pollfd[0].revents & POLLIN))
+			break;
+
+		ret = read(args->uffd, &msg, sizeof(msg));
+		if (ret != sizeof(msg))
+			break;
+
+		if (msg.event != UFFD_EVENT_PAGEFAULT)
+			break;
+
+		/* Fill the page with a known pattern */
+		memset(args->page_buffer, 0xAB, args->page_size);
+
+		copy.dst = msg.arg.pagefault.address & ~(args->page_size - 1);
+		copy.src = (unsigned long)args->page_buffer;
+		copy.len = args->page_size;
+		copy.mode = 0;
+		copy.copy = 0;
+
+		ret = ioctl(args->uffd, UFFDIO_COPY, &copy);
+		if (ret < 0)
+			break;
+	}
+
+	return NULL;
+}
+
+TEST_F(hmm, userfaultfd_read)
+{
+	struct hmm_buffer *buffer;
+	struct uffd_thread_args uffd_args;
+	unsigned long npages;
+	unsigned long size;
+	unsigned long i;
+	unsigned char *ptr;
+	pthread_t thread;
+	int uffd;
+	int stop_fd;
+	int ret;
+	struct uffdio_api api;
+	struct uffdio_register reg;
+	uint64_t stop = 1;
+	ssize_t nwrite;
+
+	npages = 4;
+	size = npages << self->page_shift;
+
+	/* Create userfaultfd */
+	uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
+	if (uffd < 0)
+		SKIP(return, "userfaultfd not available");
+
+	api.api = UFFD_API;
+	api.features = 0;
+	ret = ioctl(uffd, UFFDIO_API, &api);
+	ASSERT_EQ(ret, 0);
+
+	buffer = malloc(sizeof(*buffer));
+	ASSERT_NE(buffer, NULL);
+
+	buffer->fd = -1;
+	buffer->size = size;
+	buffer->mirror = malloc(size);
+	ASSERT_NE(buffer->mirror, NULL);
+
+	/* Create anonymous mapping */
+	buffer->ptr = mmap(NULL, size,
+			   PROT_READ | PROT_WRITE,
+			   MAP_PRIVATE | MAP_ANONYMOUS,
+			   -1, 0);
+	ASSERT_NE(buffer->ptr, MAP_FAILED);
+
+	/* Register the region with userfaultfd */
+	reg.range.start = (unsigned long)buffer->ptr;
+	reg.range.len = size;
+	reg.mode = UFFDIO_REGISTER_MODE_MISSING;
+	ret = ioctl(uffd, UFFDIO_REGISTER, &reg);
+	ASSERT_EQ(ret, 0);
+
+	/* Set up the handler thread */
+	uffd_args.uffd = uffd;
+	stop_fd = eventfd(0, EFD_CLOEXEC);
+	ASSERT_GE(stop_fd, 0);
+	uffd_args.stop_fd = stop_fd;
+	uffd_args.page_buffer = malloc(self->page_size);
+	ASSERT_NE(uffd_args.page_buffer, NULL);
+	uffd_args.page_size = self->page_size;
+
+	ret = pthread_create(&thread, NULL, uffd_handler_thread, &uffd_args);
+	ASSERT_EQ(ret, 0);
+
+	/*
+	 * Use the unlocked read path which allows the mmap lock to be
+	 * dropped during the fault, enabling userfaultfd resolution.
+	 */
+	ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_READ_UNLOCKED,
+			      buffer, npages);
+	ASSERT_EQ(ret, 0);
+	ASSERT_EQ(buffer->cpages, npages);
+
+	/* Verify the device read the data filled by the uffd handler */
+	ptr = buffer->mirror;
+	for (i = 0; i < size; ++i)
+		ASSERT_EQ(ptr[i], (unsigned char)0xAB);
+
+	nwrite = write(stop_fd, &stop, sizeof(stop));
+	ASSERT_EQ(nwrite, sizeof(stop));
+	pthread_join(thread, NULL);
+	close(stop_fd);
+	free(uffd_args.page_buffer);
+	close(uffd);
+	hmm_buffer_free(buffer);
+}
+
 TEST_HARNESS_MAIN

[PATCH v6 4/4] mshv: Use hmm_range_fault_unlocked() for region faults

[Stanislav Kinsburskii]

Convert mshv_region_hmm_fault_and_lock() to use
hmm_range_fault_unlocked() instead of taking mmap_read_lock() around
hmm_range_fault() directly.

This lets the HMM helper own the mmap read lock and allows the MSHV fault
path to handle mappings whose fault handlers may drop mmap_lock, including
userfaultfd-backed VMAs. The existing caller already retries on -EBUSY
after refreshing the mmu_interval_notifier sequence, so no control flow
change is needed beyond using the unlocked helper.

Signed-off-by: Stanislav Kinsburskii <skinsburskii@gmail.com>
---
 drivers/hv/mshv_regions.c |   14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/drivers/hv/mshv_regions.c b/drivers/hv/mshv_regions.c
index 6d65e5b42152..04676f06c5c7 100644
--- a/drivers/hv/mshv_regions.c
+++ b/drivers/hv/mshv_regions.c
@@ -388,13 +388,11 @@ int mshv_region_get(struct mshv_mem_region *region)
  *
  * This function performs the following steps:
  * 1. Reads the notifier sequence for the HMM range.
- * 2. Acquires a read lock on the memory map.
- * 3. Handles HMM faults for the specified range.
- * 4. Releases the read lock on the memory map.
- * 5. If successful, locks the memory region mutex.
- * 6. Verifies if the notifier sequence has changed during the operation.
+ * 2. Handles HMM faults for the specified range.
+ * 3. If successful, locks the memory region mutex.
+ * 4. Verifies if the notifier sequence has changed during the operation.
  *    If it has, releases the mutex and returns -EBUSY to match with
- *    hmm_range_fault() return code for repeating.
+ *    hmm_range_fault_unlocked() so the caller retries the range fault.
  *
  * Return: 0 on success, a negative error code otherwise.
  */
@@ -404,9 +402,7 @@ static int mshv_region_hmm_fault_and_lock(struct mshv_mem_region *region,
 	int ret;
 
 	range->notifier_seq = mmu_interval_read_begin(range->notifier);
-	mmap_read_lock(region->mreg_mni.mm);
-	ret = hmm_range_fault(range);
-	mmap_read_unlock(region->mreg_mni.mm);
+	ret = hmm_range_fault_unlocked(range);
 	if (ret)
 		return ret;
 

Re: [PATCH v6 2/4] mm/hmm: add hmm_range_fault_unlocked() for mmap lock-drop support

[Jason Gunthorpe]

On Mon, Jul 06, 2026 at 10:55:07AM -0700, Stanislav Kinsburskii wrote:

> +The caller must not hold ``mmap_read_lock`` before the call.
> +``hmm_range_fault_unlocked()`` takes the mmap read lock internally and allows
> +``handle_mm_fault()`` to drop it during fault handling. If the mmap lock is
> +dropped, the function returns ``-EBUSY``. The caller must then restart the
> +walk from ``range->start`` with a fresh notifier sequence. ``-EINTR`` is
> +returned if a fatal signal is pending during retry handling.

It doesn't make sense to me to expose this to the caller, the caller
isn't holding the lock and there is no reason to redo the mmu_interval
at this point, so why not just loop inside hmm_range_fault_unlocked ?

Jason

Re: [PATCH v6 4/4] mshv: Use hmm_range_fault_unlocked() for region faults

[Jason Gunthorpe]

On Mon, Jul 06, 2026 at 10:55:21AM -0700, Stanislav Kinsburskii wrote:
> Convert mshv_region_hmm_fault_and_lock() to use
> hmm_range_fault_unlocked() instead of taking mmap_read_lock() around
> hmm_range_fault() directly.

Please convert all the trivial users too, thanks

Jason