KBuffers are now zero-filled on demand instead of up front. This means
that you can create a huge KBuffer and it will only take up VM, not
physical pages (until you access them.)
Instead of generating ByteBuffers and keeping those lying around, have
these filesystems generate KBuffers instead. These are way less spooky
to leave around for a while.
Since FileDescription will keep a generated file buffer around until
userspace has read the whole thing, this prevents trivially exhausting
the kmalloc heap by opening many files in /proc for example.
The code responsible for generating each /proc file is not perfectly
efficient and many of them still use ByteBuffers internally but they
at least go away when we return now. :^)
A KBuffer always contains a valid KBufferImpl. If you need a "null"
state buffer, use Optional<KBuffer>.
This makes KBuffer very easy to work with and pass around, just like
ByteBuffer before it.
This memory is not accessible to userspace and comes from the kernel
page allocator, not from the kmalloc heap. This makes it ideal for
larger allocations.
