  In the string holographic dual of large-N_c QCD with N_f flavours of
Kruczenski et al, the eta' meson is massless at infinite N_c and dual to a
collective fluctuation of N_f D6-brane probes in a supergravity background.
Here we identify the string diagrams responsible for the generation of a mass
of order N_f/N_c, consistent with the Witten-Veneziano formula, and show that
the supregravity limit of these diagrams corresponds to mixings with
pseudoscalar glueballs. We argue that the dependence on the theta-angle in the
supergravity description occurs only through the combination theta + 2
\sqrt{N_f} eta' / f_pi, as dictated by the U(1) anomaly. We provide a
quantitative test by computing the linear term in the eta' potential in two
independent ways, with perfect agreement.
