I was reading John Rennie's answer here, Can light travel faster?
I now know that the sentence ''the speed of light is always constant in vacuum'' is just an oversimplification, for example light travel at the speed greater than the speed of light (non-locally) sometime and also can travel at the speed less than the speed of light (non-locally).
Its just when in special relativity the metric tensor take its simple form light travel at the speed of light.(SR is local). [sir John Rennie's example: if you were hovering just outside the event horizon you'd observe the speed of light to be less than $c$ everywhere nearer the black hole than you, but faster than $c$ everywhere farther from the black hole than you.]
Doesn't it mean information travelling faster than light (at least non-locally)? Is it possible? And also how much local is local?