Re: How to switch between installed kernel and developed kernel

From: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>
To: Gidi Gal <gidi.gal.linux@gmail.com>
Cc: Aruna Hewapathirane <aruna.hewapathirane@gmail.com>,
	kernelnewbies@kernelnewbies.org
Subject: Re: How to switch between installed kernel and developed kernel
Date: Mon, 22 Mar 2021 21:17:13 -0400	[thread overview]
Message-ID: <323555.1616462233@turing-police> (raw)
In-Reply-To: <CAB+0Vo=m9TM838=uUHTLPdsf1pTomvcfoqdBUFC_t-HivhJ7Wg@mail.gmail.com>

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On Tue, 23 Mar 2021 00:01:22 +0200, Gidi Gal said:

> Many thanks for your reply, Aruna. Is there a way to remove the installed
> '5.12.0-rc3-GIDI_DEV+' kernel ? A reverse command for the 'sudo make
> modules_install install' command ? I found this link which explains how to
> do it manually (
> https://www.cyberciti.biz/faq/debian-redhat-linux-delete-kernel-command/),
> I wonder if there is a safer way.

I can't speak for Debian, but I've used both the RedHat rpm method and just
using the 'rm' command for self-bullt kernels since the 2.5.47 kernel or so
(Egads, that was November 2002).  As long as you follow the directions, you
should be OK.  'rm' can get dangerous if you get over-exuberant with using '*'
characters, but you already knew that, right? :)

And if you followed my recommendation and back up /boot, you'll be all
set to restore whatever you mess up.  The running kernel will work just fine
as long as you don't reboot. And unless you did 'rm /boot/*', you should have
at least one usable kernel left...

Seriously - if you're not comfortable with that level of sysadmin procedures,
maybe you shouldn't be a kernel hacker... there is always the possibility of
something you didn't know about trashing your system.  See 5.12.0-rc1-dontuse
for a nasty bug with file-backed swap that would stomp all over a section of your
filesystem, and there was an ext[34] (can't remember anymore) bug during 2.5
that would trash the filesystem when you *unmounted* it.  So you could boot the
new kernel for testing, shutdown and boot the older version, and find it
won't boot and be blaming the older version until we figured out what was
happening. :)

But seriously - if you have a good backup of the system, and an bootable
external image that you can use for rescue, there's not much a kernel screw-up
can do to permanently lose date.

Of course, WIndows Update is at that same level of reliability, so "knowing how
to recover a trashed system" is an important skill no matter what OS you run :)

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