linux-kernel - Re: [v2 1/5] arm64: kdump: reserve memory for crash dump kernel

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Message-ID: <5551AC97.9040802@linaro.org>
Date:	Tue, 12 May 2015 16:32:39 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Baoquan He <bhe@...hat.com>
CC:	linux-arm-kernel@...ts.infradead.org, hbabus@...ibm.com,
	linaro-kernel@...ts.linaro.org, geoff@...radead.org,
	catalin.marinas@....com, will.deacon@....com,
	linux-kernel@...r.kernel.org, broonie@...nel.org,
	david.griego@...aro.org, kexec@...ts.infradead.org,
	vgoyal@...hat.com
Subject: Re: [v2 1/5] arm64: kdump: reserve memory for crash dump kernel

On 05/11/2015 06:41 PM, Baoquan He wrote:
> On 05/11/15 at 05:17pm, AKASHI Takahiro wrote:
>> On 05/11/2015 04:54 PM, Baoquan He wrote:
>>> In this patch you reserve a separate memory region in 1st kernel to
>>> store elfcorehdr. I am wondering why you don't call add_buffer in
>>> kexec-tools directly. Like this you can get a region from reserved
>>> crashkernel region. Then you don't need reserve_elfcorehdr() to reserve
>>> memory for elfcorehdr specifically. Like other ARCHs do only one memory
>>> region is reserved in 1st kernel, that's crashkernel region.
>>
>> I think that you misunderstand somewhat.
>> * Kexec-tools only locates/identifies a small region for elfcore header within crash kernel's
>> memory region while 1st kernel is running.
>> * the data in elfcore header is filled up by kexec_load system call on 1st kernel.
>> * 1st kernel doesn't reserve any region for elfcore header because the kernel
>> commandline parameters don't contains "elfcorehdr=" parameter, then elfcorehdr_size=0.
>> * Crash dump kernel does reserve the region, as I said, because we don't want to
>> corrupt the info in elfcore header accidentally while crash kernel is running.
>>
>> Clear?
>
> OK, got it now.
>
> Then I am wondering why "elfcorehdr=" can't be contained in kernel
> cmdline as other ARCH does. Maybe I need go over all related threads
> then know why it is. Thanks for explanation.

Kexec-tools on 1st kernel appends "elfcorehdr=" to the kernel command line,
actually chosen/bootargs in a device tree, that is passed to *crash dump kernel*.
So when crash dump kernel boots up, it can recognizes that area (and reserves it
for later use of managing /proc/vmcore.)

Thanks,
-Takahiro AKASHI

> Thanks
> Baoquan
>
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