linux-kernel - Re: [PATCH] x86, kdump: Set crashkernel

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Date:	Mon, 11 Mar 2013 15:17:34 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, WANG Chao <chaowang@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, kdump: Set crashkernel_low automatically

On Mon, Mar 11, 2013 at 12:04:38PM -0700, H. Peter Anvin wrote:
> On 03/11/2013 12:02 PM, Vivek Goyal wrote:
> >>
> >> What is the purpose of reserving that kind of memory below 896 MB?  If
> >> you have a 32-bit system, it will likely be useless since you are
> >> robbing the primary of most of lowmem, on a 64-bit system 896 MB is not
> >> a magic value in any way...?
> > 
> > Actually I am not sure where did 896MB magic value had come from for
> > x86_64 so far. I assumed that it was some kexec-tools limitation so
> > first trying 896MB will preserve working with old kexec-tools. If it
> > was some kernel limitation, then I agree it should not be required anymore.
> > 
> > I do remember that old pugatory had 2G limit. So may be we can first
> > try reserve with-in first 2G, then with-in first 4G and then above
> > 4G. (Assuming 896M was not kexec-tools limitation and had something
> > to do with kernel/initramfs).
> > 
> 
> It is obvious where it *originated* from... it is the *default* (but not
> necessarily the actual!) HIGHMEM crossover point on x86-32.
> 

On x86-32, max addr limit is 512M. 896M limit is on x86_64. So it probably
came from somewhere else. 

Also always reserving at high memory cuts down on what kind of bzImage
can be booted from that address. For example, x86_32bit kernels. Hence
reserving at low addresses enables booting more type of images without
rebooting.

Thanks
Vivek
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