It seems that the problem with pinning is that a megablock will end up containing only a single block, leaving the space for the other blocks unused. The block can't be moved to another megablock because it is pinned, so it can't free the entire megablock.
My suggestion is to unmap the pages corresponding to the empty space in the megablock. So if a megablock only contains a single block, unmap the 255 empty pages.
