Logic behind killing processes during an Out of Memory situation

A simplified explanation of the OOM-killer logic follows.
A function called badness() is defined to calculate points for each processes. Points are added to:

• Processes with high memory usage
• Niced processes

Badness points are subtracted from:

• Processes which have been running for a long time
• Processes which were started by superusers
• Process with direct hardware access

The process with the highest number of badness points will be killed, unless it is already in the midst of freeing up memory on its own. (Note that if a processes has 0 points it can not be killed.)

The kernel will wait for some time to see if enough memory is freed by killing one process. If enough memory is not freed, the OOM-kills will continue until enough memory is freed or until there are no candidate processes left to kill. If the kernel is out of memory and is unable to find a candidate process to kill, it panics with a message like: