how to use kexec to jump back to the original kernel

[Gioh Kim]

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,
>
> Thank you for your advice.
>
> I tried load-preserve-context as below but it failed with "Invalid
> argument" error.
> There is no reason in the error message.
> But there is entry address that looks like the address "kexec -e" in
> the kdump kernel will jump to.
> Following is what I did.
>
> root at pserver:~# kexec -l /boot/vmlinuz-`uname -r`
> --initrd=/boot/initrd.img-`uname -r` --reuse-cmdline
> --load-preserve-context --mem-min=0x1000000 --mem-max=0xffffffff
> kexec_load failed: Invalid argument
> entry       = 0xbffcb7f0 flags = 0x3e0002
> nr_segments = 7
> segment[0].buf   = (nil)
> segment[0].bufsz = 0x0
> segment[0].mem   = 0x1000000
> segment[0].memsz = 0xba080000
> segment[1].buf   = 0x7fe372eaa010
> segment[1].bufsz = 0x2f7fddc
> segment[1].mem   = 0xbb080000
> segment[1].memsz = 0x2f80000
> segment[2].buf   = 0x7fe375e2e210
> segment[2].bufsz = 0x61c840
> segment[2].mem   = 0xbe000000
> segment[2].memsz = 0x14e1000
> segment[3].buf   = (nil)
> segment[3].bufsz = 0x0
> segment[3].mem   = 0xbf4e1000
> segment[3].memsz = 0xae5000
> segment[4].buf   = 0x25eb920
> segment[4].bufsz = 0x42c6
> segment[4].mem   = 0xbffc6000
> segment[4].memsz = 0x5000
> segment[5].buf   = 0x25e27c0
> segment[5].bufsz = 0x9100
> segment[5].mem   = 0xbffcb000
> segment[5].memsz = 0xb000
> segment[6].buf   = 0x25e1e20
> segment[6].bufsz = 0x400
> segment[6].mem   = 0xbffd6000
> segment[6].memsz = 0x4000
> root at pserver:~#
> root at pserver:~#
> root at pserver:~# kexec -e
> Nothing has been loaded!
>

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?


-- 

Best regards,
Gioh Kim