Message-ID: <20110707193321.GE28850@redhat.com>
Date:	Thu, 7 Jul 2011 15:33:21 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
Cc:	ebiederm@...ssion.com, hbabu@...ibm.com, mahesh@...ux.vnet.ibm.com,
	oomichi@....nes.nec.co.jp, horms@...ge.net.au,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [patch 0/9] kdump: Patch series for s390 support

On Wed, Jul 06, 2011 at 11:24:47AM +0200, Michael Holzheu wrote:
> Hello Vivec,
> 
> On Tue, 2011-07-05 at 16:26 -0400, Vivek Goyal wrote:
> > On Mon, Jul 04, 2011 at 07:09:22PM +0200, Michael Holzheu wrote:
> 
> [snip]
> 
> > I don't understand what is stand-alone dump tools and 
> 
> S390 stand-alone dump tools are independent mini operating systems that
> are installed on disks or tapes. When a dump should be created, these
> stand-alone dump tools are booted. All that they do is to write the dump
> (current memory plus the CPU registers) to the disk/tape device.
> 
> The advantage compared to kdump is that since they are freshly loaded
> into memory they can't be overwritten in memory.

> Another advantage is
> that since it is different code, it is much less likely that the dump
> tool will run into the same problem than the previously crashed kernel.

I think in practice this is not really a problem. If your kernel
is not stable enough to even boot and copy a file, then most likely
it has not even been deployed. The very fact that a kernel has been
up and running verifies that it is a stable kernel for that machine
and is capable of capturing the dump.

> Also the boot process ensures that the hardware is in a initialized
> state.

Who makes sure that hardware is in initiliazed state? Kdump kernel,
stand alone kernel or BIOS.

> And last but not least, with the stand-alone dump tools you can
> dump early kernel problems which is not possible using kdump, because
> you can't dump before the kdump kernel has been loaded with kexec.
> 

That is one limitation but again if your kernel can't even boot,
it is not ready to ship and it is more of a development issue and
there are other ways to debug problems. So I would not worry too
much about it.

On a side note, few months back there were folks who were trying
to enhance bootloaders to be able to prepare basic environment so
that a kdump kernel can boot even in the event of early first
kernel boot.

> That were more or less the arguments, why we did not support kdump in
> the past.
> 
> In order to increase dump reliability with kdump, we now implemented a
> two stage approach. The stand-alone dump tools first check via meminfo,
> if kdump is valid using checksums. If kdump is loaded and healthy it is
> started. Otherwise the stand-alone dump tools create a full-blown
> stand-alone dump.

kexec-tools purgatory code also checks the checksum of loaded kernel
and other information and next kernel boot starts only if nothing
has been corrupted in first kernel. So this additional meminfo strucutres
and need of checksums sounds unnecessary. I think what you do need is
that somehow invoking second hook (s390 specific stand alone kernel)
in case primary kernel is corrupted.

> 
> With this approach we still keep our s390 dump reliability and gain the
> great kdump features, e.g. distributor installer support, dump filtering
> with makedumpfile, etc.
> 
> > why the existing
> > mechanism of preparing ELF headers to describe all the above info
> > and just passing the address of header on kernel commnad line
> > (crashkernel=) will not work for s390. Introducing an entirely new
> > infrastructure for communicating the same information does not
> > sound too exciting.
> 
> We need the meminfo interface anyway for the two stage approach. The
> stand-alone dump tools have to find and verify the kdump kernel in order
> to start it.

kexec-tools does this verification already. We verify the checksum of
all the loaded information in reserved area. So why introduce this
meminfo interface.

> Therefore the interface is there and can be used. Also
> creating the ELF header in the 2nd kernel is more flexible and easier
> IMHO:
> * You do not have to care about memory or CPU hotplug.

Reloading the kernel upon memory or cpu hotplug should be trivial. This
does not justify to move away from standard ELF interface and creation
of a new one.

> * You do not have to preallocate CPU crash notes etc.

Its a small per cpu area. Looks like otherwise you will create meminfo
areas otherwise.

> * It works independently from the tool/mechanism that loads the kdump
> kernel into memory. E.g. we have the idea to load the kdump kernel at
> boot time into the crashkernel memory (not via the kexec_load system
> call). That would solve the main kdump problems: The kdump kernel can't
> be overwritten by I/O and also early kernel problems could then be
> dumped using kdump.

Can you give more details how exactly it works. I know very little about
s390 dump mechanism.

When do you load kdump kernel and who does it?

Who gets the control first after crash?

To me it looked like that you regularly load kdump kernel and if that
is corrupted then somehow you boot standalone kernel. So corruption
of kdump kernel should not be a issue for you.

Do you load kdump kenrel from some tape/storage after system crash. Where
does bootloader lies and how do you make sure it is not corrupted and
associated device is in good condition.

To me we should not create a arch specific way of passing information
between kernels. Stand alone kernel should be able to parse the
ELF headers which contains all the relevant info. They have already
been checksum verified.

Thanks
Vivek
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