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

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Date:	Fri, 17 Dec 2010 13:35:07 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Maxim Uvarov <muvarov@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Neil Horman <nhorman@...hat.com>
Subject: Re: kdump broken on 2.6.37-rc4

On Fri, Dec 17, 2010 at 10:21:24AM -0800, Yinghai Lu wrote:
> On 12/17/2010 10:02 AM, Vivek Goyal wrote:
> > On Fri, Dec 17, 2010 at 09:56:42AM -0800, H. Peter Anvin wrote:
> >> On 12/17/2010 09:01 AM, Vivek Goyal wrote:
> >>> On Thu, Dec 16, 2010 at 07:58:48PM -0800, Yinghai wrote:
> >>>> Please don't do that to 64 bit
> >>>>
> >>>> My big system with 1024g memory and a lot of cards with rhel 6 to make kdump work must have crashkernel=512m and second kernel need to take pci=nomsi
> >>>>
> >>>
> >>> I agree here that we should not do it for 64 bit.
> >>>
> >>> - Just because we need it for 32 bit does not mean we should limit it for
> >>>   64bit. And we do want to have the capability to boot the kernel from as
> >>>   high memory as possible so creating another aritificial limit is counter
> >>>   to that.
> >>>
> >>> - I would not worry too much about backward compatibility and allow
> >>>   booting 32bit kernel till 768MB. The reason being that most of the
> >>>   distros use same kernel for crash dumping as regular kernel. Maintainig
> >>>   two separate kernels is big hassle.
> >>>
> >>>   So a small set of people who run into issue, would need to change kernel
> >>>   command line "crashkernel=128M@64M" or something similar.
> >>>
> >>
> >> Do we have actual testing for how high the 64-bit kernel will load?
> > 
> > I will do some experiments on my box today and let you know.
> 
> if bzImage is used, it is 896M.

Strangely on my x86_84 systems with 37-rc6, I am trying to reserve memory and
nothing shows up on /proc/iomem. dmesg says that I am reaserving 128M at 64M
but nothing in /proc/iomeme. Going back to .36 kernel and see what
happens.

Ok, last time we had looked that kexec-tools had constraint to load
bzImage and initrd below 896MB and it must be coming from that.
 
> 
> or crashkernel=... will take two ranges like one high and one low.
> 
> also kexec bzImage in 64bit should use startup_64 aka 0x200 offset instead of startup_32 in arch/x86/boot/compressed/head_64.S
> 
> then bzImage can be put above 4G...

Neil had been trying that but AFAIK, he had no success. I don't know but
he was struggling with setting up pages tables in kexec for 64bit startup.

But yes, making use of 64bit entry point is in the wish list.

Thanks
Vivek
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