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School of Computer Science & Engineering
University of New South Wales

 Advanced Operating Systems 
 COMP9242 2017/S2 

M3: A pager

Use your memory manager from M2 to write a simple VM fault handler that supports on-demand memory mapping of an application and an allocate-on-demand memory heap.

Current Implementation

The current implementation simply pre-maps in the pages for the single binary. Any actual VM page faults will trigger an assert() which halts the system, under the assumption something has gone wrong. The executable itself is mapped in with untracked frames (i.e. we leak them), and finally, the current sos application malloc implementation uses a static array.

A small malloc() intro

'malloc' is the standard library function to allocate memory in C. In our system, malloc is provided by the musl libc library. malloc manages memory from a bigger memory pool. In the SOS code you are provided, malloc uses memory from a pre-allocated array in the initial data section as the memory pool. This pool is fixed in size, and malloc returns NULL when the pool is exhausted. See the diagram below for an approximate picture of the memory layout. The code musl uses to allocate memory from the static region for SOS is in apps/sos/src/sys/sys_morecore.c

SOS applications use an independent implementation that uses a pre-allocated memory pool in the application's data section (as shown in the middle of the diagram). The code musl libc uses in applications is in libs/libsos/src/sys_morecore.c. So just for emphasis, there are two versions of the morecore code in the system.

One of the tasks of this milestone is to leverage virtual memory to create a dynamically allocated memory region for malloc's use, usually termed the heap, as shown at the bottom of the diagram. The dynamic region is allocated on-demand via VM faults, and can be expanded in range dynamically by requesting that SOS increase the brk point.

In this milestone, you will modify the application-level morecore routines in libs/libsos/src/sys_morecore.c to use your dynamically-allocated memory region. Again, be sure you're modifying the morecore routines for sos applications, not SOS's own morecore routines.

Malloc and musl libc peculiarities

The malloc implementation in musl libc (i.e. the application C library) has two behaviours for implementing memory allocation of the memory pool:

The Milestone

In this milestone you will:

Design alternatives

Probably the main thing that you should consider here is the layout of your processes address space. Some things you will want to consider is where you place various parts of memory such as the stack, heap and code segments. You may also have some other regions in your process address space, one of these is the IPC Buffer.

You should also think about if you want to make different ranges of the address space have different permissions, eg: you may want to make code read-only to prevent bugs, or have a guard page at the end of your stack to prevent overflow.

While not needed for this milestone, you should think about what book keeping is required to delete an address space and free all the resources associated with it.



The main demonstration here will be to show a user process running with a high stack pointer (> 0x20000000). You should also demonstrate a user process using malloc() from a heap.

#include <utils/page.h>

#define NPAGES 27
#define TEST_ADDRESS 0x20000000

/* called from pt_test */
static void
do_pt_test(char *buf)
    int i;

    /* set */
    for (int i = 0; i < NPAGES; i++) {
	    buf[i * PAGE_SIZE_4K] = i;

    /* check */
    for (int i = 0; i < NPAGES; i++) {
	    assert(buf[i * PAGE_SIZE_4K] == i);

static void
pt_test( void )
    /* need a decent sized stack */
    char buf1[NPAGES * PAGE_SIZE_4K], *buf2 = NULL;

    /* check the stack is above phys mem */
    assert((void *) buf1 > (void *) TEST_ADDRESS);

    /* stack test */

    /* heap test */
    buf2 = malloc(NPAGES * PAGE_SIZE_4K);

You should also be able to explain to the tutor how your code works and any design decisions you took.

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Last modified: 04 Aug 2017.