One of the kernel’s primary jobs is to manage the memory installed in the
system. Over the years, though, there have been various reasons for
removing a portion of the system’s memory from the kernel’s view. One of
the latest can be seen in a mechanism called DMEMFS,
which is being proposed as a way to get around some inefficiency in how the
kernel keeps track of RAM.

Source: LWN.net – [$] Sidestepping kernel memory management with DMEMFS