This change looks safe to me. I was just not sure that it is enough for practical cases. What if after allocating a large buffer the rest of the new area would be used for allocating small buffers? They can keep references to the large area after freeing the large buffer. Perhaps it is worth to block marking the remainder of a large area available. This will increase the memory consumption by small percent, but will reduce the risk of prolonging the life time of large blocks.