Mon Apr 28 16:12:25 1997
Stefan Savage & Yasushi Saito
Overview
This core SPIN memory management services are
designed to provide access to both physical memory resources and the
MMU. These services are necessary (and ideally sufficient) to develop
general virtual memory systems as well as specialized memory
management implementations.
It is equally important that these services guard against any
misuse or malicious action. Particularly, it must be impossible to
modify kernel code or data structure in any way which might compromise
the type-safety properties required by the kernel. Secondarily, the
kernel must guard against resource consumption attacks
(e.g. allocating undue quantities of physical memory). Spincore
addresses the first problem by encapsulating each page frame in an opaque
Modula-3 object(PhysAddr.T), and allowing accesses to frame contents
only through procedure calls. The second problem is addressed by
implementing a involuntary frame purging through the use of
the dispatcher mechanism.
Basic abstractions
SPIN's basic memory abstractions are physical frames
(PhysAddr), and translations to virtual addresses
(Translation). In general, access to memory consists of
two steps: allocating some physical frame and mapping it to a virtual
address range in a particular translation context.
Physical Addresses
The PhysAddr.i3
interface defines all public operations on physical memory.
Also, it defines the opaque data type
PhysAddr.T which represents a contiguous block of
physical memory. There are two kinds of PhysAddr.T objects. One is
an ordinally page frame of the MMU page size. Other is an object that
represents arbitrary physical address range. The latter is used to
access I/O space, and it can be allocated only through
PhysAddrPrivate.AllocateIOSpace, a
privileged procedure.
Translations
A translation represents a context in which virtual address are
translated (eg. an address space or protection domain). It is
currently implemented in terms of pmap operations. The
Translation.i3
interface defines all operations on translations. It also defines the
opaque data type Translation.T which represents a particular
translation context.
Getting and setting VM parameters
The vm shell command is provided to tweak the vm parameters.
-
vm params
- This command returns the current parameters.
I don't know what the request threshold is.
If the # of free pages fall below reclaim threshold,
an active page is picked up and reclaimed.
If the # of free pages fall below repossess threshold,
an reclaimed page is picked up and made free.
-
vm setparams
- This command takes three integer parameters. Each number
specifies the request, reclaim, repossess threshold repectively.
-
vm stat
- This command returns the # of pages in each state.
Memory Management