linux-kernel - Re: kdump broken on 2.6.37-rc4

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Date:	Wed, 15 Dec 2010 11:39:54 +0100
From:	Stanislaw Gruszka <sgruszka@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Yinghai Lu <yinghai@...nel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Maxim Uvarov <muvarov@...il.com>, linux-kernel@...r.kernel.org,
	Neil Horman <nhorman@...hat.com>
Subject: Re: kdump broken on 2.6.37-rc4

On Tue, Dec 14, 2010 at 05:41:36PM -0500, Vivek Goyal wrote:
> On Mon, Dec 13, 2010 at 11:47:08AM -0800, H. Peter Anvin wrote:
> > On 12/13/2010 10:20 AM, Yinghai Lu wrote:
> > > 
> > > it seems 32bit kdump need crashkernel much low than we expect...
> > > 
> > > Maybe we have to find_in_range_low() to make 32bit kdump happy.
> > > 
> > 
> > Not this garbage again... sigh.  Once again, I will want to know what
> > the actual constraint is... not just "oh, this seems to work on this one
> > system."
> > 
> > I realize that the kdump interfaces are probably beyond saving -- we
> > have had this discussion enough times -- but I'm not happy about it and
> > I will really want to know what the heck the real issue is.
> 
> Same here Yinghai. We need to debug that what is that upper limit for
> loading x86 32bit kernel and if we know/understand that, we can fail
> the loading of kdump kernel citing the appropriate reason. Last time
> our understanding was that as long as we allocate memory below 896MB
> things should be fine.
> 
> Stanislaw, how much memory you are reserving at what address with -rc4
> kernel? 

crashkernel=128M, system has 1G mem.

> Can you please look at  /proc/iomem? And try to reserve same
> amount of memory at roughly same address at 2.6.36 kernel, and see if
> kdump works.
> 
> So how I used to debug problems in kdump path. 
> 
> - Try earlyprintk for second kernel.
> - Try --debug, --console-serial options with kexec while loading second
>   kernel. Important thing to know here is control reached to purgatory
>   or not.
> - If that gives me nothing then it boils down to putting some outb()
>   statements in first kernel and second kernel boot path to know where
>   things went wrong.
> 
>   Because the issue was resolved by reserving memory in low memory
>   area, it sounds like second kernel failed to boot early. So early
>   printk might help otherwise outb() and serial console is the friend.

I could debug this problem, but I do not suffer from free time right
now :-) Would be better someone bootmem/kdump experienced debug this.
I just check other laptop (T500, 2.6.37-rc5, x86_64, RHEL6 user space,
crashkernel=256M, 1.6G mem), kdump does not work there too. So I do
think problem is hard to reproduce.

Stanislaw
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