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Lok
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Joined: 23 Mar 2006
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PostPosted: Thu Sep 14, 2017 5:23 pm    Post subject: /proc/vmcore not found Reply with quote

How to dump kernel log?

# cat /proc/vmcore
cat: /proc/vmcore: No such file or directory

# cat /proc/cmdline
BOOT_IMAGE=/boot/kernel-genkernel-x86_64-4.12.5-gentoo root=UUID=b5d861b7-8985-40c4-953d-3eefd34624b6 ro crashkernel=64M

# zcat /proc/config.gz |grep -i kexec
CONFIG_KEXEC_CORE=y
CONFIG_KEXEC=y
# CONFIG_KEXEC_JUMP is not set
# zcat /proc/config.gz |grep -i vmcore
CONFIG_PROC_VMCORE=y
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eccerr0r
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PostPosted: Thu Sep 14, 2017 6:38 pm    Post subject: Reply with quote

I'm not sure I understand what you're asking...

vmcore is the crash dump of the last kernel that you used, as recovered by kexec'ed recovery kernel. This is for fairly advanced users, but I can't tell if this is what you want by your question. I wouldn't call it a "log", more of a crash memory dump.

If you mean old kernel-generated text warning/error logs, it depends on what you've or what the kernel could have saved. If you have syslogd installed, usually what's emitted from the kernel is logged in /var/log/messages. For systemd, the command journalctl has everything saved.
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Lok
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Joined: 23 Mar 2006
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PostPosted: Thu Sep 14, 2017 8:13 pm    Post subject: Reply with quote

Ok,
how to dump kernel by kexec?

# cat /proc/cmdline
BOOT_IMAGE=/boot/kernel-genkernel-x86_64-4.12.5-gentoo root=UUID=b5d861b7-8985-40c4-953d-3eefd34624b6 ro crashkernel=64M


# kexec -p /boot/kernel-genkernel-x86_64-4.12.5-gentoo --initrd=/boot/initramfs-genkernel-x86_64-4.12.5-gentoo --append="root=/dev/sda3 single irqpoll maxcpus=1 reset_devices dolvm softlevel=kdump"
Unknown type (Reserved) while parsing /sys/firmware/memmap/17/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/7/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/5/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/3/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/21/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/1/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/18/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/16/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/14/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/20/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/10/type. Please report this as bug. Using RANGE_RESERVED now.
Unknown type (Reserved) while parsing /sys/firmware/memmap/19/type. Please report this as bug. Using RANGE_RESERVED now.
ELF core (kcore) parse failed
Cannot load /boot/kernel-genkernel-x86_64-4.12.5-gentoo
#
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eccerr0r
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PostPosted: Thu Sep 14, 2017 9:00 pm    Post subject: Reply with quote

The kexec is done by the kernel itself when it detects a crash. If everything is setup properly, just cause a kernel crash (could sysrq-c) and it should kexec the recovery kernel and the vmdump will show up.
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Lok
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PostPosted: Thu Sep 14, 2017 9:45 pm    Post subject: Reply with quote

Not work

kernel config https://pastebin.com/GrGKiaEW

# cat /etc/conf.d/kexec|grep -vE "^#"
ROOTPART="/dev/sda3"
KNAME="kernel-4"
INITRD="initramfs-4"
DONT_MOUNT_BOOT="yes"

# cat /etc/kexec.conf|grep -vE "^#"
KNAME="kernel-4"
KEXEC_OPT_ARGS="--initrd=initramfs-4"

# cat /proc/cmdline
BOOT_IMAGE=/boot/kernel-genkernel-x86_64-4.12.5-gentoo root=UUID=b5d861b7-8985-40c4-953d-3eefd34624b6 ro crashkernel=64M

But after reboot start new kernel
rc.log: kexec | * Using kernel image /boot/kernel-4 (with /boot/initramfs-4) for kexec ...

# cat /sys/kernel/kexec_loaded
0

# /etc/init.d/kexec restart
kexec | * WARNING: you are stopping a boot service
kexec | * Not rebooting; disabling kexec ... [ ok ]

sysrq-c -- system freeze, and kernel not start.
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eccerr0r
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PostPosted: Thu Sep 14, 2017 10:29 pm    Post subject: Reply with quote

Okay I can't explain things better than https://wiki.gentoo.org/wiki/Kernel_Crash_Dumps
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Lok
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PostPosted: Fri Sep 15, 2017 10:13 am    Post subject: Reply with quote

Thank you,
Apparently something with kexec, because I can load and start the kernel fine with the -l and -e options, but -p doesn’t seem to trigger.
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