[$] Memory: the flat, the discontiguous, and the sparse

The physical memory in a computer system is a precious resource, so a
lot of effort has been put into managing it effectively. This task is made
more difficult by the complexity of the memory architecture on contemporary
systems. There are several
layers of abstraction that deal with the details of how physical memory
is laid out; one of those is simply called the "memory model". There
are three models supported in the kernel, but one of them is on its way
out. As a way of understanding this change, this article will
take a closer look at the evolution of the kernel's memory models,
their current state, and their possible future.