We have reanalyzed the giant outburst of the blazar PKS 1510−089 (z = 0.36) that occurred on 2011 October−November. The γ-ray flux in the 0.1−100 GeV energy range exceeded the value of 10-5 ph cm-2 s-1 for several days. The peak flux was reached on 2011 October 19, with a value of ~4.4 × 10-5 ph cm-2 s-1, which in turn corresponds to a luminosity of ~2 × 1049 erg s-1. A very short timescale variability was measured. Particularly on 2011 October 18, the flux-doubling time was as short as ~20 min. This is the shortest variability ever detected in the MeV-GeV energy band. We compared our analysis with two other outbursts observed in 2009 March and 2012 February−March, when the blazar was also detected by H.E.S.S. and MAGIC to infer information about the emission at hundreds of GeV.