diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 3ca50fbb1..dfd134b77 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -123,7 +123,7 @@ hb_blob_create_sub_blob (hb_blob_t    *parent,
 
   blob = hb_blob_create (parent->data + offset,
 			 MIN (length, parent->length - offset),
-			 parent->mode,
+			 HB_MEMORY_MODE_READONLY,
 			 hb_blob_reference (parent),
 			 (hb_destroy_func_t) hb_blob_destroy);
 
diff --git a/src/hb-blob.h b/src/hb-blob.h
index 1a93baa46..d3d0f41b1 100644
--- a/src/hb-blob.h
+++ b/src/hb-blob.h
@@ -36,6 +36,26 @@
 HB_BEGIN_DECLS
 
 
+/*
+ * Note re various memory-modes:
+ *
+ * - In no case shall the HarfBuzz client modify memory
+ *   that is passed to HarfBuzz in a blob.  If there is
+ *   any such possibility, MODE_DUPLICATE should be used
+ *   such that HarfBuzz makes a copy immediately,
+ *
+ * - Use MODE_READONLY otherse, unless you really really
+ *   really know what you are doing,
+ *
+ * - MODE_WRITABLE is appropriate if you relaly made a
+ *   copy of data solely for the purpose of passing to
+ *   HarfBuzz and doing that just once (no reuse!),
+ *
+ * - If the font is mmap()ed, it's ok to use
+ *   READONLY_MAY_MAKE_WRITABLE, however, there were
+ *   design problems with that mode, so HarfBuzz do not
+ *   really use it anymore.  If not sure, use MODE_READONLY.
+ */
 typedef enum {
   HB_MEMORY_MODE_DUPLICATE,
   HB_MEMORY_MODE_READONLY,
@@ -52,6 +72,12 @@ hb_blob_create (const char        *data,
 		void              *user_data,
 		hb_destroy_func_t  destroy);
 
+/* Always creates with MEMORY_MODE_READONLY.
+ * Even if the parent blob is writable, we don't
+ * want the user of the sub-blob to be able to
+ * modify the parent data as that data may be
+ * shared among multiple sub-blobs.
+ */
 hb_blob_t *
 hb_blob_create_sub_blob (hb_blob_t    *parent,
 			 unsigned int  offset,