MadPinguin has posted a guide about upgrading to kernel 2.4.21:

This HOWTO will describe the procedures involved in upgrading your Linux kernel to the latest stable release (currently 2.4.21). Even though this will work with any Linux distribution, it was written and designed for use on Slackware Linux 9.0 running a stock 2.4.20 kernel. Use at your own risk and try not to blow up your machine! If you do, don't call me.

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