The Design and Implementation of the Anykernel and Rump Kernels

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to secondary storage so that memory can be reclaimed for other purposes. Instead of
requiring a MMU to keep track of page usage, we observe that since memory pages
allocated from a rump kernel cannot be mapped into a client’s address space, the
pages are used only in kernel code. Every time kernel code wants to access a page,
it does a lookup for it using uvm_pagelookup(), uses it, and releases the reference.
Therefore, we hook usage information tracking to the lookup routine: whenever a
lookup is done, the page is deemed as accessed.
3.4.1 Page Remapping
A regular NetBSD kernel has the ability to interact with the MMU and map physical
pages to virtual addresses 8 . Since a rump kernel allocates only anonymous memory
from the host, it cannot ask the host to remap any of its allocations at least on a
POSIX system — there is no interface for mapping anonymous memory in multiple
places. Usermode operating systems typically use a memory mapped file to represent
the physical memory of the virtual kernel [26, 31]. The file acts as a handle and
can be mapped to the location(s) desired by the usermode kernel using the mmap()
system call. The DragonFly usermode vkernel uses special host system calls to make
the host kernel execute low level mappings [31].
Using a file-backed solution is in conflict with the lightweight fundamentals of rump
kernels. First, the file must be created at a path specified either by a configuration
option or by a system guess. Furthermore, it disrupts the dynamic memory principle
we have laid out. While it is relatively simple to extend the file to create more
memory on demand, it is difficult to truncate the file in case the memory is no longer
needed because free pages will not automatically reside in a contiguous range at
the end of the file. Finally, the approach requires virtual memory mapping support
from the host and limits the environments a rump kernel can be theoretically hosted
8 NetBSD itself does not run on hardware without a MMU.