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

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Message-ID: <513E2AC6.1000707@zytor.com>
Date:	Mon, 11 Mar 2013 12:04:38 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vivek Goyal <vgoyal@...hat.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 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.

Whether this limitation has crept into somewhere else is a good question.

	-hpa


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