diff --git a/Documentation/vm/hmm.rst b/Documentation/vm/hmm.rst
index 44205f0b671f..d9b27bdadd1b 100644
--- a/Documentation/vm/hmm.rst
+++ b/Documentation/vm/hmm.rst
@@ -189,11 +189,7 @@ the driver callback returns.
 When the device driver wants to populate a range of virtual addresses, it can
 use either::
 
-  int hmm_vma_get_pfns(struct vm_area_struct *vma,
-                      struct hmm_range *range,
-                      unsigned long start,
-                      unsigned long end,
-                      hmm_pfn_t *pfns);
+  long hmm_range_snapshot(struct hmm_range *range);
   int hmm_vma_fault(struct vm_area_struct *vma,
                     struct hmm_range *range,
                     unsigned long start,
@@ -202,7 +198,7 @@ When the device driver wants to populate a range of virtual addresses, it can
                     bool write,
                     bool block);
 
-The first one (hmm_vma_get_pfns()) will only fetch present CPU page table
+The first one (hmm_range_snapshot()) will only fetch present CPU page table
 entries and will not trigger a page fault on missing or non-present entries.
 The second one does trigger a page fault on missing or read-only entry if the
 write parameter is true. Page faults use the generic mm page fault code path
@@ -220,19 +216,33 @@ Locking with the update() callback is the most important aspect the driver must
  {
       struct hmm_range range;
       ...
+
+      range.start = ...;
+      range.end = ...;
+      range.pfns = ...;
+      range.flags = ...;
+      range.values = ...;
+      range.pfn_shift = ...;
+
  again:
-      ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
-      if (ret)
+      down_read(&mm->mmap_sem);
+      range.vma = ...;
+      ret = hmm_range_snapshot(&range);
+      if (ret) {
+          up_read(&mm->mmap_sem);
           return ret;
+      }
       take_lock(driver->update);
       if (!hmm_vma_range_done(vma, &range)) {
           release_lock(driver->update);
+          up_read(&mm->mmap_sem);
           goto again;
       }
 
       // Use pfns array content to update device page table
 
       release_lock(driver->update);
+      up_read(&mm->mmap_sem);
       return 0;
  }
 
diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index 716fc61fa6d4..32206b0b1bfd 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -365,11 +365,11 @@ void hmm_mirror_unregister(struct hmm_mirror *mirror);
  * table invalidation serializes on it.
  *
  * YOU MUST CALL hmm_vma_range_done() ONCE AND ONLY ONCE EACH TIME YOU CALL
- * hmm_vma_get_pfns() WITHOUT ERROR !
+ * hmm_range_snapshot() WITHOUT ERROR !
  *
  * IF YOU DO NOT FOLLOW THE ABOVE RULE THE SNAPSHOT CONTENT MIGHT BE INVALID !
  */
-int hmm_vma_get_pfns(struct hmm_range *range);
+long hmm_range_snapshot(struct hmm_range *range);
 bool hmm_vma_range_done(struct hmm_range *range);
 
 
diff --git a/mm/hmm.c b/mm/hmm.c
index 84e0577a912a..bd957a9f10d1 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -702,23 +702,25 @@ static void hmm_pfns_special(struct hmm_range *range)
 }
 
 /*
- * hmm_vma_get_pfns() - snapshot CPU page table for a range of virtual addresses
- * @range: range being snapshotted
- * Returns: -EINVAL if invalid argument, -ENOMEM out of memory, -EPERM invalid
- *          vma permission, 0 success
+ * hmm_range_snapshot() - snapshot CPU page table for a range
+ * @range: range
+ * Returns: number of valid pages in range->pfns[] (from range start
+ *          address). This may be zero. If the return value is negative,
+ *          then one of the following values may be returned:
+ *
+ *           -EINVAL  invalid arguments or mm or virtual address are in an
+ *                    invalid vma (ie either hugetlbfs or device file vma).
+ *           -EPERM   For example, asking for write, when the range is
+ *                    read-only
+ *           -EAGAIN  Caller needs to retry
+ *           -EFAULT  Either no valid vma exists for this range, or it is
+ *                    illegal to access the range
  *
  * This snapshots the CPU page table for a range of virtual addresses. Snapshot
  * validity is tracked by range struct. See hmm_vma_range_done() for further
  * information.
- *
- * The range struct is initialized here. It tracks the CPU page table, but only
- * if the function returns success (0), in which case the caller must then call
- * hmm_vma_range_done() to stop CPU page table update tracking on this range.
- *
- * NOT CALLING hmm_vma_range_done() IF FUNCTION RETURNS 0 WILL LEAD TO SERIOUS
- * MEMORY CORRUPTION ! YOU HAVE BEEN WARNED !
  */
-int hmm_vma_get_pfns(struct hmm_range *range)
+long hmm_range_snapshot(struct hmm_range *range)
 {
 	struct vm_area_struct *vma = range->vma;
 	struct hmm_vma_walk hmm_vma_walk;
@@ -772,6 +774,7 @@ int hmm_vma_get_pfns(struct hmm_range *range)
 	hmm_vma_walk.fault = false;
 	hmm_vma_walk.range = range;
 	mm_walk.private = &hmm_vma_walk;
+	hmm_vma_walk.last = range->start;
 
 	mm_walk.vma = vma;
 	mm_walk.mm = vma->vm_mm;
@@ -788,9 +791,9 @@ int hmm_vma_get_pfns(struct hmm_range *range)
 	 * function return 0).
 	 */
 	range->hmm = hmm;
-	return 0;
+	return (hmm_vma_walk.last - range->start) >> PAGE_SHIFT;
 }
-EXPORT_SYMBOL(hmm_vma_get_pfns);
+EXPORT_SYMBOL(hmm_range_snapshot);
 
 /*
  * hmm_vma_range_done() - stop tracking change to CPU page table over a range
