Re: Direct dumped kernel cores

On Thu, Oct 31, 2024, 7:07 PM Justin Hibbits <jhibbits@freebsd.org> wrote:

> On Thu, 31 Oct 2024 15:33:39 -0700
> Ravi Pokala <rpokala@freebsd.org> wrote:
>
> > Hi Justin,
> >
> > So, this is like the 'crashkernel' thing Linux has, where it kexec()s
> > an alternate kernel when the main one panics?
>
> If your description is accurate, then probably.  I don't know if the
> crashkernel thing from Linux existed when this was started, and
> actually hadn't even heard of it until now.
>

Yeah. There's three types of kernels: debug, crash, and replacement.

>
> > I haven't looked at the patch -- most of it will be way out of my
> > expertise -- but what, if anything, is done to make sure the on-disk
> > state of the target filesystem is okay, before the "rescue" kernel
> > starts writing to it?
>
> Good question.  The rescue kernel embeds its own small rootfs it fscks
> the target fs before writing to it.
>

Yea. My loader.kboot loads the kernel and initrd too...

Warner


> - Justin
>
> >
> > Thanks,
> >
> > Ravi (rpokala@)
> >
> > -----Original Message-----
> > From: <owner-freebsd-arch@FreeBSD.org
> > <mailto:owner-freebsd-arch@FreeBSD.org>> on behalf of Justin Hibbits
> > <jhibbits@FreeBSD.org <mailto:jhibbits@FreeBSD.org>> Date: Thursday,
> > October 31, 2024 at 15:23 To: <freebsd-hackers@FreeBSD.org
> > <mailto:freebsd-hackers@FreeBSD.org>>, <freebsd-arch@freebsd.org
> > <mailto:freebsd-arch@freebsd.org>> Subject: Direct dumped kernel cores
> >
> >
> > Hi everyone,
> >
> >
> > At Juniper we've been using a so-called 'rescue' kernel for dumping
> > vmcores directly to the filesystem after a panic. We're now
> > contributing this feature, implemented by Klara Systems, to FreeBSD,
> > and looking for feedback. I posted a review
> > at https://reviews.freebsd.org/D47358
> > <https://reviews.freebsd.org/D47358> for anyone interested.
> >
> >
> > Interesting bits to keep in mind:
> > * It requires a 2-stage build process, one to build the rescue kernel,
> > the other to build the main kernel, which embeds the rescue kernel
> > inside its image. This might need some further work.
> > * Thus far it's been implemented for amd64 and arm64, once proven out,
> > other architectures (powerpc64/le, riscv64) can follow suit.
> > * Kernel environment bits to pass down to the rescue kernel are
> > prefixed `debug.rescue.`, for instance
> > `debug.rescue.vfs.root.mountfrom`.
> >
> >
> > There are many more details in the review summary.
> >
> >
> > We'd love to get feedback from anyone interested.
> >
> >
> > Thanks,
> > Justin Hibbits
> >
> >
> >
> >
> >
> >
>
>
>