Although it seems unlikely, this could raise a `ValueError` if a line happens to have a different number of fields.  I haven't seen any case where `/proc/mounts` didn't contain exactly 6 fields by line, but I know we've been burned in other places where the number of fields in some `/proc` files would sometimes vary with exotic things configured.  So maybe we should wrap this line with a `try` .. `except ValueError` and log the error and a `continue` to skip the bad line.