Demystifying Process Address Space: Heap, Stack, and Beyond

We all have a vague idea about how the heap, stack, and code section are laid out in process virtual address space. But what does it actually look like? Does the heap always grow downwards? What is vDSO? How does address space layout randomization come into play? How do JIT compilers put code in memory and execute it? What else is mapped into the address space of programs?