CISS-150 Project 6 – Swap/Paging Space

CISS-150 Project 6 (10 points)

(Updated April 28, 2025)

Overview

The Ubuntu VM provided to you has no swap/paging space. Examination through the use of free and top will reveal this.

Ubuntu 17.04 and later allows the use of a file in the standard filesystem to provide the storage needed for paging space. You will go through a series of steps to create the paging space, confirm the system will use it, make it permanent, and confirm that it survives reboots.

Learning outcomes

• Planning and design.
• Modifying OS memory management resources.
• Understanding swap/paging space.
• Understanding /etc/fstab.

Getting Started

Swap space (also called paging space since physical memory pages are written to disk) is an extension of physical memory. However, for a program to run, it must be in physical memory. Swap space does not mean you have more memory to run programs; you have a place to put running program data and code to get it out of the way so other work can be done.

Your Ubuntu system is intentionally lacking swap space. The first step is to confirm there is no swap space.

student@student-vm:~$ sudo -i
[sudo] password for student: 
root@student-vm:~# swapon --show
root@student-vm:~# free
              total        used        free      shared  buff/cache   available
Mem:        3036072      737268     1675676        7600      623128     2121536
Swap:             0           0           0
root@student-vm:~# 

Note that we became the root user, so all the commands are run as root.

Now we will create a 2GB disk file in the root directory for use as swap space

root@student-vm:~# fallocate -l 2G /swapfile
root@student-vm:~# ll /swapfile
-rw-r--r-- 1 root root 2147483648 Dec 29 15:11 /swapfile
root@student-vm:~# 

Configuring Swap Space

Now that we have the file in place, there are a few tweaks that are necessary to use it as a swap space. The first is to set the permissions correctly, then we need to format the file for use as swap space.

First, the file permissions must be set such that only the root user can write to it.

root@student-vm:~# chmod 600 /swapfile

Now we format the space as a swap file.

root@student-vm:~# mkswap /swapfile 
Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
no label, UUID=fd567bb2-abe0-4093-8b96-9b3be2d4ce0e

At this point, we can try turning on the swap space.

root@student-vm:~# swapon /swapfile
root@student-vm:~# swapon --show
NAME      TYPE SIZE USED PRIO
/swapfile file   2G   0B   -2

Recall that earlier, we saw no swap space available when we used free to report all available memory. Rerunning it after the swap space has been activated will reveal it has changed.

root@student-vm:~# free
              total        used        free      shared  buff/cache   available
Mem:        3036072     1079940     1059240       58336      896892     1725384
Swap:       2097148           0     2097148
root@student-vm:~# 

Adding Persistence

The /etc/fstab file is the text file that defines all available filesystems within the OS. It will look something like the following:

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
#                
/dev/mapper/vgubuntu-root /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/sda1 during installation
UUID=B587-A388  /boot/efi       vfat    umask=0077      0       1

There are six fields separated by whitespace. For more information on the fields themselves, you can run

root@student-vm:~# man fstab

This file maintains information about storage that needs to be available at boot time. Part of the boot process is to mount known filesystems so that the files and directories they contain will be available to the rest of the OS, software packages, and system users.

Before we go any further, we will make a backup copy of the /etc/fstab file:

root@student-vm:~# cp /etc/fstab /etc/fstab.orig
root@student-vm:~# ll /etc/fstab*
-rw-rw-r-- 1 root root 543 Dec 29 15:45 /etc/fstab
-rw-r--r-- 1 root root 543 Dec 29 15:50 /etc/fstab.orig
root@student-vm:~#

This way, if we mess up when we do the confirmation later, we can go back without losing all of your work.

Using your editor of choice (nano, gedit, vi, etc.), add the line at the bottom of the file /etc/fstab.

/swapfile none swap sw 0 0

You can also run something like the following:

root@student-vm:~# echo "/swapfile none swap sw 0 0" | tee -a /etc/fstab
root@student-vm:~# cat /etc/fstab

The file will now look like the following:

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
#                
/dev/mapper/vgubuntu-root /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/sda1 during installation
UUID=B587-A388  /boot/efi       vfat    umask=0077      0       1
/swapfile none swap sw 0 0

If your file looks drastically different, DO NOT REBOOT! Perform the recovery procedure below and try again.

If all looks good, then go ahead and reboot. When the system is back up, log in and check the swap status and free memory to confirm the swap is persistent across boots.

Recovery

If your resultant file does not look like the original plus the new line, you can recover the file with the following:

root@student-vm:~# cp /etc/fstab.orig /etc/fstab

Contact your instructor before reverting the snapshot if all copies of the /etc/fstab file are wrong. They will advise you on the next steps.

Further Study

You have been assigned readings regarding swap and paging space. Be sure to review those for a deeper understanding of the purposes and limitations of swap space.

There is also an excellent guide in the Ubuntu SwapFaq.

In addition read the complete virtual memory details of OSTEP-Chapter 23.

Open a second terminal window, run top, and place the window conspicuously so you can watch the data change. Make a note of the amount of memory used, free, and buff/cache. Also, make a note of the amount of swap space used.

Experiment with the following code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

// 1 GB 
#define SIZE 1024*1024*1024
int main(int argc, char **argv) {

	void *ptr;
	char s[5];

	ptr = calloc(SIZE, sizeof(char));
	if ( ptr ) {
		memset(ptr, 'A', SIZE);
		printf("1GB allocated (%p). Press ENTER to release.\n", ptr);
	} else
		printf("An error occurred (%d). Press ENTER.\n", errno);

	fgets(s, 4, stdin);
}

Save it as mem.c and perform the following:

root@student-vm:~# gcc -o mem mem.c

Now, before you run the program, consider this: The use of calloc() forces the initialization of the memory block to zero. However, modern Linuxes already zero the pages, so the pages do not get mapped and perform this allocation in a lazy way similar to malloc(). As a result, we have to use memset() to force the pages to be mapped.

root@student-vm:~# ./mem
1GB allocated. Press ENTER to release.
root@student-vm:~# 

It will be obvious that memory values change in top. Since we are using calloc() and memset(), the memory pages must be touched in order to set them to zero.

Two things to note about Linux is

1. It is a demand-paging system.
2. Memory allocation defaults to lazy allocation.

Regarding #1, demand-paging is basically only mapping pages to a process’ page table when they are actually touched. See TLDP’s documentation memory management. Specifically 3.1, 3.1.1, 3.1.2, 3.2, 3.4, 3.6, 3.8.

Regarding #2, since malloc() explicitly states it does not initialize the memory, then it simply hands back a pointer to memory that is in use from the heap’s perspective, but not allocated by the kernel since the pages have not yet been touched.

Now, this means that the system is allowed to overcommit memory resources. This allows the system to maintain a high level of multiprogramming since it is playing the odds that not all programs will use all of the requested memory. This is a logical approach to resource management and has been applied to many areas of computing, especially networking (i.e., a switch with 48-1Gb ports and a 10Gb uplink. This is overcommitted 4.8 to 1.)

A couple of values in the Linux system manage memory overcommit. There are noted here.

root@student-vm:~# cat /proc/sys/vm/overcommit_memory 
0
root@student-vm:~# cat /proc/sys/vm/overcommit_ratio 
50
root@student-vm:~# 

Details of how the three different values of vm.overcommit_memory are defined at the Kernel website.