how to use kexec to jump back to the original kernel

[Gioh Kim]

On Tue, Feb 13, 2018 at 3:36 PM, Gioh Kim <gurugio at gmail.com> wrote:
> On Tue, Feb 13, 2018 at 11:06 AM, Gioh Kim <gurugio at gmail.com> wrote:
>> On Mon, Feb 12, 2018 at 7:56 AM, Bhupesh SHARMA <bhupesh.linux at gmail.com> wrote:
>>> Hello,
>>>
>>> On Fri, Feb 9, 2018 at 7:24 PM, Gioh Kim <gurugio at gmail.com> wrote:
>>>> Hello,
>>>>
>>>> "echo c > /proc/sysrq-trigger" generates the kernel dump but it
>>>> reboots the system.
>>>> I'd like to generate kernel dump without system reboot.
>>>>
>>>> I think it is possible to jump to the kdump kernel and generate kernel
>>>> dump, and come back to the operating kernel with kexec.
>>>> I know I can jump to the kdump kernel with "kexec -e" but I cannot
>>>> figure out how to come back to the operating kernel.
>>>>
>>>> For example, I tried following sequences after reading kexec manual page.
>>>>
>>>> 0. rebuild kernel with CONFIG_KEXEC_JUMP option
>>>> 1. kexec --load=/boot/vmlinuz --initrd=/boot/initrd --reuse-cmdline
>>>
>>> Instead you should be using something like:
>>> # kexec -l /boot/vmlinuz-`uname -r` --initrd=/boot/initramfs-`uname
>>> -r`.img --reuse-cmdline  --load-preserve-context --mem-min=addr
>>> --mem-max=addr
>>
>> Hello Bhupesh,
>
> I'm sorry to bother you.
> I failed because I booted up wrong kernel without KEXEC_JUMP.
> I can jump to kdump kernel with booting a kernel with KEXEC_JUMP.
> And I can see "kexec_jump_back_entry=0x00000000000810d2" at kernel
> booting parameter.
> Now I can jump to kdump kernel and jump back to the original kernel.
>
> Can I ask just one more question?
> After jumping to kdump kernel, there is not /proc/vmcore file.
> I checked the kernel enabled CONFIG_PROC_VMCORE option.
> What kernel option should I enable to use /proc/vmcore?
> Or should I use another file instead /proc/vmcore?
>
>

I'm sorry but please let me ask one more question.

I ran "kexec -e" command on kdump kernel, then it just rebooted the system.
I guess jumping to the original kernel means resuming the original
kernel as it was running.
Did I misunderstand?
Or my machine did not work correctly?


-- 

Best regards,
Gioh Kim