linux-kernel - Re: Kexec query about what makes sure control pages/page tables are not overwritten

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Date:	Mon, 11 Nov 2013 12:57:12 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Vivek Goyal <vgoyal@...hat.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: Kexec query about what makes sure control pages/page tables are not overwritten

It is pretty simple: the kernel exports how much text+data+bss+brk it needs, and the kernel cannot use memory outside that region until it is ready to control the address space itself.

Vivek Goyal <vgoyal@...hat.com> wrote:
>Hi,
>
>I am staring at control page allocation logic in case of kdump and
>wondering what makes sure that these pages are not overwritten by 
>next kernel.
>
>I see that for 64bit entry, control pages have the page tables needed
>for second kernel. In case of crash these pages from from crash kernel
>reserved region. Page allocator is very simple and that is start from
>lowest crash reserved range and move higher and allocate first
>available
>page which is not allocated to segments. What makes sure that these
>pages
>are not overwritten by second kernel.
>
>I guess it becomes a general bootloader question. How do we make sure
>bootloader prepared page tables/gdt will not be overwritten by kernel
>(till kernel sets up its own page tables and gdt) and how should we do
>the
>allocation and placement.
>
>Thanks
>Vivek

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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