  Let $(X, Y)$ be a suitable couple of quasi-Banach lattices of measurable
functions on $\mathbb T \times \Omega$, and let $(X_A, Y_A)$ be the couple of
the corresponding Hardy-type spaces. It has long been suspected that the
BMO-regularity property of $(X, Y)$ is not only sufficient for the $\mathrm
K$-closedness of $(X_A, Y_A)$ in $(X, Y)$ but also necessary. We establish the
equivalence of these two properties for a general couple of Banach lattices
having the Fatou property when $\Omega$ is a discrete measurable space, and
also for couples $(X, Y)$ where $X$ is allowed to be quasi-Banach but $Y$ is
assumed to be $p$-convex with some $p > 1$ (here $\Omega$ is arbitrary). We
show under certain mild restrictions that the "good interpolation" formula $$
\left(X_A, \mathrm H_q\right)_{\theta, p} = \left[\left(X, \mathrm
L_q\right)_{\theta, p}\right]_A $$ holds true if and only if $X$ is
BMO-regular.
