diff --git a/src/include/rbd/librbd.hpp b/src/include/rbd/librbd.hpp
index d061bdbe4eb86..600a3ccbcda00 100644
--- a/src/include/rbd/librbd.hpp
+++ b/src/include/rbd/librbd.hpp
@@ -161,22 +161,23 @@ class Image
    *
    * This will return the differences between two versions of an image
    * via a callback, which gets the offset and length and a flag
-   * indicating whether the extent is known/defined to be zeros (a
-   * hole).  If the source snapshot name is NULL, we interpret that as
-   * the beginning of time and return all allocated regions of the
-   * image.  The end version is whatever is currently selected for the
-   * image handle (either a snapshot or the writeable head).
+   * indicating whether the extent exists (1), or is known/defined to
+   * be zeros (a hole, 0).  If the source snapshot name is NULL, we
+   * interpret that as the beginning of time and return all allocated
+   * regions of the image.  The end version is whatever is currently
+   * selected for the image handle (either a snapshot or the writeable
+   * head).
    *
    * @param fromsnapname start snapshot name, or NULL
    * @param ofs start offset
    * @param len len in bytes of region to report on
    * @param cb callback to call for each allocated region
    * @param arg argument to pass to the callback
-   * @returns len on success, or negative error code on error
+   * @returns 0 on success, or negative error code on error
    */
-  int64_t diff_iterate(const char *fromsnapname,
-		       uint64_t ofs, size_t len,
-		       int (*cb)(uint64_t, size_t, bool, void *), void *arg);
+  int diff_iterate(const char *fromsnapname,
+		   uint64_t ofs, size_t len,
+		   int (*cb)(uint64_t, size_t, int, void *), void *arg);
   ssize_t write(uint64_t ofs, size_t len, ceph::bufferlist& bl);
   int discard(uint64_t ofs, uint64_t len);
 
diff --git a/src/librbd/internal.cc b/src/librbd/internal.cc
index 4daa5b801db3d..49573966f8ba3 100644
--- a/src/librbd/internal.cc
+++ b/src/librbd/internal.cc
@@ -2269,18 +2269,22 @@ namespace librbd {
     return total_read;
   }
 
-  int simple_diff_cb(uint64_t off, size_t len, bool exists, void *arg)
+  int simple_diff_cb(uint64_t off, size_t len, int exists, void *arg)
   {
+    // This reads the existing extents in a parent from the beginning
+    // of time.  Since images are thin-provisioned, the extents will
+    // always represent data, not holes.
+    assert(exists);
     interval_set<uint64_t> *diff = static_cast<interval_set<uint64_t> *>(arg);
     diff->insert(off, len);
     return 0;
   }
 
 
-  int64_t diff_iterate(ImageCtx *ictx, const char *fromsnapname,
-		       uint64_t off, uint64_t len,
-		       int (*cb)(uint64_t, size_t, bool, void *),
-		       void *arg)
+  int diff_iterate(ImageCtx *ictx, const char *fromsnapname,
+		   uint64_t off, uint64_t len,
+		   int (*cb)(uint64_t, size_t, int, void *),
+		   void *arg)
   {
     utime_t start_time, elapsed;
 
@@ -2291,8 +2295,7 @@ namespace librbd {
     if (r < 0)
       return r;
 
-    uint64_t mylen = len;
-    r = clip_io(ictx, off, &mylen);
+    r = clip_io(ictx, off, &len);
     if (r < 0)
       return r;
 
@@ -2347,9 +2350,8 @@ namespace librbd {
 	return r;
     }
 
-    int64_t total_read = 0;
     uint64_t period = ictx->get_stripe_period();
-    uint64_t left = mylen;
+    uint64_t left = len;
 
     while (left > 0) {
       uint64_t period_off = off - (off % period);
@@ -2385,7 +2387,7 @@ namespace librbd {
 		o.intersection_of(parent_diff);
 		ldout(ictx->cct, 20) << " reporting parent overlap " << o << dendl;
 		for (interval_set<uint64_t>::iterator s = o.begin(); s != o.end(); ++s) {
-		  cb(s.get_start(), s.get_len(), false, arg);
+		  cb(s.get_start(), s.get_len(), true, arg);
 		}
 	      }
 	    }
@@ -2431,7 +2433,7 @@ namespace librbd {
 				   << " logical "
 				   << logical_off << "~" << s.get_len()
 				   << dendl;
-	      cb(logical_off, s.get_len(), !end_exists, arg);
+	      cb(logical_off, s.get_len(), end_exists, arg);
 	    }
 	    opos += r->second;
 	  }
@@ -2439,12 +2441,11 @@ namespace librbd {
 	}
       }
 
-      total_read += read_len;
       left -= read_len;
       off += read_len;
     }
 
-    return total_read;
+    return 0;
   }
 
   int simple_read_cb(uint64_t ofs, size_t len, const char *buf, void *arg)
diff --git a/src/librbd/internal.h b/src/librbd/internal.h
index 7f66006e12c43..e407d44af9e0d 100644
--- a/src/librbd/internal.h
+++ b/src/librbd/internal.h
@@ -168,10 +168,10 @@ namespace librbd {
   int64_t read_iterate(ImageCtx *ictx, uint64_t off, size_t len,
 		       int (*cb)(uint64_t, size_t, const char *, void *),
 		       void *arg);
-  int64_t diff_iterate(ImageCtx *ictx, const char *fromsnapname,
-		       uint64_t off, uint64_t len,
-		       int (*cb)(uint64_t, size_t, bool, void *),
-		       void *arg);
+  int diff_iterate(ImageCtx *ictx, const char *fromsnapname,
+		   uint64_t off, uint64_t len,
+		   int (*cb)(uint64_t, size_t, int, void *),
+		   void *arg);
   ssize_t read(ImageCtx *ictx, uint64_t off, size_t len, char *buf);
   ssize_t read(ImageCtx *ictx, const vector<pair<uint64_t,uint64_t> >& image_extents,
 	       char *buf, bufferlist *pbl);
diff --git a/src/librbd/librbd.cc b/src/librbd/librbd.cc
index b522288b6a7d9..20dbf62393b45 100644
--- a/src/librbd/librbd.cc
+++ b/src/librbd/librbd.cc
@@ -442,10 +442,10 @@ namespace librbd {
     return librbd::read_iterate(ictx, ofs, len, cb, arg);
   }
 
-  int64_t Image::diff_iterate(const char *fromsnapname,
-			      uint64_t ofs, uint64_t len,
-			      int (*cb)(uint64_t, size_t, bool, void *),
-			      void *arg)
+  int Image::diff_iterate(const char *fromsnapname,
+			  uint64_t ofs, uint64_t len,
+			  int (*cb)(uint64_t, size_t, int, void *),
+			  void *arg)
   {
     ImageCtx *ictx = (ImageCtx *)ctx;
     return librbd::diff_iterate(ictx, fromsnapname, ofs, len, cb, arg);
@@ -1032,6 +1032,16 @@ extern "C" int64_t rbd_read_iterate(rbd_image_t image, uint64_t ofs, size_t len,
   return librbd::read_iterate(ictx, ofs, len, cb, arg);
 }
 
+extern "C" int diff_iterate(rbd_image_t image,
+			    const char *fromsnapname,
+			    uint64_t ofs, uint64_t len,
+			    int (*cb)(uint64_t, size_t, int, void *),
+			    void *arg)
+{
+  librbd::ImageCtx *ictx = (librbd::ImageCtx *)image;
+  return librbd::diff_iterate(ictx, fromsnapname, ofs, len, cb, arg);
+}
+
 extern "C" ssize_t rbd_write(rbd_image_t image, uint64_t ofs, size_t len,
 			     const char *buf)
 {
diff --git a/src/rbd.cc b/src/rbd.cc
index f6cdb58390517..d776ef018cdf6 100644
--- a/src/rbd.cc
+++ b/src/rbd.cc
@@ -1035,14 +1035,14 @@ static int do_export(librbd::Image& image, const char *path)
   return r;
 }
 
-static int export_diff_cb(uint64_t ofs, size_t _len, bool zero, void *arg)
+static int export_diff_cb(uint64_t ofs, size_t _len, int exists, void *arg)
 {
   ExportContext *ec = static_cast<ExportContext *>(arg);
   int r;
 
   // extent
   bufferlist bl;
-  __u8 tag = zero ? 'z' : 'w';
+  __u8 tag = exists ? 'w' : 'z';
   ::encode(tag, bl);
   ::encode(ofs, bl);
   uint64_t len = _len;
@@ -1051,7 +1051,7 @@ static int export_diff_cb(uint64_t ofs, size_t _len, bool zero, void *arg)
   if (r < 0)
     return r;
 
-  if (!zero) {
+  if (exists) {
     // read block
     bl.clear();
     r = ec->image->read(ofs, len, bl);
@@ -1071,7 +1071,7 @@ static int do_export_diff(librbd::Image& image, const char *fromsnapname,
 			  const char *endsnapname,
 			  const char *path)
 {
-  int64_t r;
+  int r;
   librbd::image_info_t info;
   int fd;
 
@@ -1139,7 +1139,7 @@ static int do_export_diff(librbd::Image& image, const char *fromsnapname,
   return r;
 }
 
-static int diff_cb(uint64_t ofs, size_t len, bool zero, void *arg)
+static int diff_cb(uint64_t ofs, size_t len, int exists, void *arg)
 {
   cout << ofs << "\t" << len << "\t"
        << (zero ? "zero" : "data") << "\n";
@@ -1148,7 +1148,7 @@ static int diff_cb(uint64_t ofs, size_t len, bool zero, void *arg)
 
 static int do_diff(librbd::Image& image, const char *fromsnapname)
 {
-  int64_t r;
+  int r;
   librbd::image_info_t info;
 
   r = image.stat(info, sizeof(info));
diff --git a/src/test/librbd/test_librbd.cc b/src/test/librbd/test_librbd.cc
index ff083ae817f48..c24f41ec95343 100644
--- a/src/test/librbd/test_librbd.cc
+++ b/src/test/librbd/test_librbd.cc
@@ -1503,7 +1503,7 @@ TEST(LibRBD, FlushAioPP)
 }
 
 
-int iterate_cb(uint64_t off, size_t len, bool zero, void *arg)
+int iterate_cb(uint64_t off, size_t len, int exists, void *arg)
 {
   cout << "iterate_cb " << off << "~" << len << std::endl;
   interval_set<uint64_t> *diff = static_cast<interval_set<uint64_t> *>(arg);
@@ -1564,7 +1564,7 @@ TEST(LibRBD, DiffIterate)
     cout << " wrote " << two << std::endl;
 
     interval_set<uint64_t> diff;
-    ASSERT_EQ((int)size, image.diff_iterate("one", 0, size, iterate_cb, (void *)&diff));
+    ASSERT_EQ(0, image.diff_iterate("one", 0, size, iterate_cb, (void *)&diff));
     cout << " diff was " << diff << std::endl;
     if (!two.subset_of(diff)) {
       interval_set<uint64_t> i;
@@ -1620,7 +1620,7 @@ TEST(LibRBD, DiffIterateStress)
 	cout << "from " << i << " to " << j << " diff " << diff << std::endl;
 
 	image.snap_set(snap[j].c_str());
-	ASSERT_EQ((int)size, image.diff_iterate(snap[i].c_str(), 0, size, iterate_cb, (void *)&actual));
+	ASSERT_EQ(0, image.diff_iterate(snap[i].c_str(), 0, size, iterate_cb, (void *)&actual));
 	cout << " actual was " << actual << std::endl;
 	if (!diff.subset_of(actual)) {
 	  interval_set<uint64_t> i;