(2019) Chinese gaokao

Let the domain of the function $f$ be all the real numbers. It is known that for all $x$ in this domain, $f(x+1) = 2f(x)$ . Also, for $x \in (0, 1]$ , $f(x) = x^2 - x$ . If $f(x) \geq -8/9$ for $x \in (-\infty, m]$ , find $3m$ .