Re: How to switch between installed kernel and developed kernel

From: Aruna Hewapathirane <aruna.hewapathirane@gmail.com>
To: "Valdis Klētnieks" <valdis.kletnieks@vt.edu>
Cc: Gidi Gal <gidi.gal.linux@gmail.com>,
	kernelnewbies <kernelnewbies@kernelnewbies.org>
Subject: Re: How to switch between installed kernel and developed kernel
Date: Tue, 23 Mar 2021 00:29:10 -0400	[thread overview]
Message-ID: <CAFSeFg8vgumYtVB+VvcYwaK1KMRpGU2Xn8jgcx2edfULh4D0Uw@mail.gmail.com> (raw)
In-Reply-To: <323555.1616462233@turing-police>

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On Mon, Mar 22, 2021 at 9:17 PM Valdis Klētnieks <valdis.kletnieks@vt.edu>
wrote:

> 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 ?

Yes there are 'many' ways to remove a 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.
>

Type linux into your shell then press the 'tab' key twice.. you will see a
list of commands.

Use : linux-check-removal <uname-r of your kernel to remove>

Once you are comfortable with compiling + linking/building +running a
custom kernel
what is preventing you from writing 'your own command' to do exactly that ?
Say a bash
script ? Or Python program ?

> 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? :)
>

If you have to use rm always use it with the -i flag. Always prompt before
removal.

> And if you followed my recommendation and back up /boot, you'll be all
> set to restore whatever you mess up.

Listen to Valdis in this case and follow orders religioulsy. Back up not
just /boot but anything
and everything that is important for you.

> 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...

Do not listen to Valdis in this case as we were all newbies at one time
like Dan Carpenter told me
which I will remember to my dying day. Do not let anyone tell you what you
can or cannot do when it
come's to the kernel because believe me like me you will find out over time
the kernel is a living thing that
has very subtle ways of informing you when you did something and it is not
happy :-)

So compile away to your hearts content and go ahead and break things like I
did that is actually a very good way to learn.
And listen to more experienced folk like Valdis who probably knows more
about all the subsystems than anyone. But if anyone tells you
you should not be a kernel hacker then prove them wrong ? Actually that is
Valdis's way of motivating you.

So good luck and we are here if you have questions :-)

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.
>

Agreed 110%.

>
> 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 :)
>

 Hope this helps - Aruna

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