-system call
-writes any modified pages back to the filesystem and updates
-the file modification time.
-If
-.Fa len
-is 0, all modified pages within the region containing
-.Fa addr
-will be flushed;
-if
-.Fa len
-is non-zero, only those pages containing
+system call writes modified whole pages back to the filesystem
+and updates the file modification time.
+Only those pages containing