linux-kernel - Re: Query about kdump_msg hook into crash

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Date:	Thu, 3 Feb 2011 21:50:42 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Américo Wang <xiyou.wangcong@...il.com>
Cc:	Seiji Aguchi <seiji.aguchi@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Jarod Wilson <jwilson@...hat.com>
Subject: Re: Query about kdump_msg hook into crash_kexec()

On Fri, Feb 04, 2011 at 10:24:27AM +0800, Américo Wang wrote:
> On Thu, Feb 03, 2011 at 05:08:01PM -0500, Seiji Aguchi wrote:
> >Hi Eric,
> >
> >Thank you for your prompt reply.
> >
> >I would like to consider "Needs in enterprise area" and "Implementation of kmsg_dump()" separately.
> >
> >(1) Needs in enterprise area
> >  In case of kdump failure, we would like to store kernel buffer to NVRAM/flush memory
> >  for detecting root cause of kernel crash.
> >
> >(2) Implementation of kmsg_dump 
> >  You suggest to review/test cording of kmsg_dump() more.
> >
> >What do you think about (1)?
> >Is it acceptable for you?
> >
> 
> For "in case of kdump fails", we can move
> KMSG_DUMP_OOPS/KMSG_DUMP_PANIC before calling crash_kexec(),
> so you can capture the log before kdump happens.

True. If the idea is to capture the logs before kdump starts saving
core, then kdump_msg() call, can be put before crash_kexec() call and
there is no need to hook into crash_kexec().

Secondly now the question of whether kdump_msg() call should be before
crash_kexec() or not because modules might want to do lots of unreliable
things, I am now split on that. Especially because of the fact that if
somebody wants it probably can use kprobe to hook into crash_kexec()
or panic() or something like that to execute its code before everything
else.

The other reason I am little split on this because in the past I have seen
kdump fail many a times either because of chipset issues or because of driver
issues etc. So though I don't like it and I want drivers to be fixed
to take care of booting in kdump environment, things not necessarily
worked as well as we expected them to and it is not hard to meet a kdump
failure.

Hence though I do not prefer it but I will not stand in the way of kdump_msg()
being called before crash_kexec() gets the control.

But we should atleast remove the hook from crash_kexec() and get rid
of additional config option (KMSG_DUMP_KEXEC).

Thanks
Vivek
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