SYNC_FILE_RANGE_WRITE can block on writes greater than request

queue size.  For some background, see
http://thread.gmane.org/gmane.linux.kernel/687713/focus=688340

man2/sync_file_range.2

@@ -70,6 +70,8 @@ before performing any write.
 .B SYNC_FILE_RANGE_WRITE
 Initiate write-out of all dirty pages in the specified
 range which are not presently submitted write-out.
+Note that even this may may block if you attempt to
+write more than request queue size.
 .TP
 .B SYNC_FILE_RANGE_WAIT_AFTER
 Wait upon write-out of all pages in the range