This section describes how I got Linux and FreeBSD to share a swap partition. There may be other ways to get the same result. You can install FreeBSD before Linux if you want to, just pay attention to the order of the partitions in the FreeBSD slice.
The first step is to install Linux as normal. You have to leave space for the FreeBSD slice at your hard drive. You don't have to make a Linux swap partition, but if you want one, put it in the space you want to allocate for FreeBSD. That way you can delete the Linux swap partition later and use the space for FreeBSD.
When you have installed Linux you have to build a new kernel. Read The Linux Kernel HOWTO if this is new to you. You have to include both UFS filesystem support (read only) and BSD disklabel (FreeBSD partition tables) support:
UFS filesystem support (read only) (CONFIG_UFS_FS) [N/y/m/?] y BSD disklabel (FreeBSD partition tables) support (CONFIG_BSD_DISKLABEL) [N/y/?] (NEW) y
Install the new kernel and reboot. Remove any line including the word swap
from your /etc/fstab file if you have made a Linux swap partition.
Make sure you have a working Linux boot floppy with the new kernel. Now
you are ready to install FreeBSD.
Install FreeBSD as described in the FreeBSD documentation. Remove the Linux
swap partition if you have made one (you can use the FreeBSD fdisk
program.) Pay attention to the order of the partitions in the FreeBSD
slice. If you use the default labelling the second partition will be the swap
partition. Complete the installation of FreeBSD and reboot into Linux
using the new Linux boot floppy.
Run dmesg when you have booted into Linux. In the output you should see
something like this:
Partition check: hda: hda1 hda2 hda3 hda4 < hda5 hda6 hda7 hda8 >
This means that /dev/hda4 is your FreeBSD slice, while
/dev/hda5, /dev/hda6, /dev/hda7 and
/dev/hda8 are the FreeBSD partitions. If your swap partition is
the second partition in the slice, it will be /dev/hda6.
You have to put the following line into your Linux /etc/fstab file to
enable the swap partition:
/dev/hda6 none swap sw 0 0
While FreeBSD can use any type of partition as swap space, Linux needs a
special signature in the swap partition. This signature is made by mkswap.
FreeBSD ruins this signature when it uses the shared swap partition, so you
will have to run mkswap each time you boot into Linux. To do this
automagically you have to find the script that runs swapon at boot time.
In Red Hat Linux it is /etc/rc.d/rc.sysinit. Put the following line
into that file just before swapon -a:
awk -- '/swap/ && ($1 !~ /#/) { system("mkswap "$1"") }' /etc/fstab
This will run mkswap on any swap partitions in /etc/fstab every
time you boot except if they are commented out (having ``#'' as the first
character in the line.)
Run free to check out the size of the swap space when you have rebooted
into Linux. You should also reboot into FreeBSD to make sure everything works
as expected. If it does not, you have probably used the wrong partition
as swap partition. The only solution to that problem is to reinstall FreeBSD
and try again. Experience is a great teacher. :-)