This is really neat. Somehow I always assumed realloc() copied stuff instead of using the page table.

But say you have 4K page table size. You malloc() in turn a 2K object, a 256K object, and another 2K object, ending up with 2K, 256K, 2K in memory. Then your 256K is not aligned on a page boundary. If you realloc() the 256K it has to move since it's surrounded by two objects. When you do that, you'll wind up with the two pages on the end being mapped to multiple addresses. Which is actually just fine...Interesting...