[PATCH] Documentation: kdump: describe jumping to dump-capture kernel

[Gi-Oh Kim]

On Wed, Feb 14, 2018 at 2:43 AM, Dave Young <dyoung at redhat.com> wrote:
> Hi,
> On 02/13/18 at 04:22pm, Gioh Kim wrote:
>> Jumping between the system kernel and the dump-capture kernel
>> has been supported for long time but there is no description
>> how to use it. This patch adds the description how to use kexec tool
>> to jump to the dump-capture kernel and jump back to the system kernel.
>
> I do not think this should belong to kdump documentation.  There are a
> lot of choices after a vmcore saving, one can reboot, halt, even go
> ahead with real root filesystem init path.  We do not need to document
> all these in kdump.txt.
>
> Since it is a general use case not only for kdump, add more info in
> kexec man page would be better.

Hi,

Whenever I find a kernel option, I search Documentation files, and
next is google.
So I think it would be better to describe how to use a kernel option
or kernel feature in kernel document.


>
>>
>> Signed-off-by: Gioh Kim <gi-oh.kim at profitbricks.com>
>> ---
>>  Documentation/kdump/kdump.txt | 38 ++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 38 insertions(+)
>>
>> diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
>> index 51814450a7f8..35b71fef5d88 100644
>> --- a/Documentation/kdump/kdump.txt
>> +++ b/Documentation/kdump/kdump.txt
>> @@ -460,6 +460,44 @@ and the system will boot into the dump-capture kernel.
>>  For testing purposes, you can trigger a crash by using "ALT-SysRq-c",
>>  "echo c > /proc/sysrq-trigger" or write a module to force the panic.
>>
>> +Jump between the System kernel and the Dump-capture kernel
>> +===============================
>> +
>> +Without system crash, the system can jump to the dump-capture kernel.
>> +
>> +1) Enable "jump between system kernel and dump-capture kernel" support under
>> +   "Processor type and features"
>> +
>> +   CONFIG_KEXEC_JUMP=y
>> +
>> +2) Load the dump-capture kernel with --load-preserve-context and mem-max
>> +   options as following.
>> +
>> +   kexec -l <dump-capture-kernel-vmlinux-image> \
>> +   --initrd=<initrd-for-dump-capture-kernel> --args-linux \
>> +   --append="root=<root-dev> <arch-specific-options>" \
>> +   --load-preserve-context \
>> +   --mem-max=<the highest memory address addr to load code into>
>> +
>> +3) Jump to the loaded kernel
>> +
>> +   kexec -e
>> +
>> +Now the system is running with the dump-capture kernel. You can jump back
>> +to the system kernel.
>> +
>> +1) Find kexec_jump_back_entry address in kernel booting parameters in
>> +   /proc/cmdline. That is the address for kexec to jump to. For example:
>> +   kexec_jump_back_entry=0x00000000000810d2
>> +
>> +2) Following command sets the jump-back address for kexec.
>> +
>> +   kexec --load-jump-back-helper --entry=0x810d2
>> +
>> +3) Jump to the system kernel
>> +
>> +   kexec -e
>> +
>>  Write Out the Dump File
>>  =======================
>>
>> --
>> 2.11.0
>>
>
> Thanks
> Dave



-- 
GIOH KIM
Linux Kernel Entwickler

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