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Chapter 17. Enabling kdump

For your RHEL 9 systems, you can configure enabling or disabling the kdump functionality on a specific kernel or on all installed kernels. However, you must routinely test the kdump functionality and validate its working status.

17.1. Enabling kdump for all installed kernels
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Start the kdump service for all kernels installed on the machine. The service starts after the kexec tool is installed.

Prerequisites

  • You have administrator privileges.

Procedure

  1. Add the crashkernel= command-line parameter to all installed kernels: 

    # grubby --update-kernel=ALL --args="crashkernel=xxM"
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    xxM is the required memory in megabytes.

  2. In terms of UKI, add the crashkernel= command-line parameter to all installed UKIs by copying an add-on associated with the required argument from /lib/modules/$(uname -r)/vmlinuz-virt.efi.extra.d/ to /boot/efi/loader/addons/.

  3. Reboot the system: 

    # reboot
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  4. Enable the kdump service: 

    # systemctl enable --now kdump.service
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Verification

  • Check that the kdump service is running: 

    # systemctl status kdump.service
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    kdump.service - Crash recovery kernel arming
     Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: disabled)
     Active: active (live)
    Copy to Clipboard Toggle word wrap

17.2. Enabling kdump for a specific installed kernel
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Start the kdump service for a specific kernel on the machine.

Prerequisites

  • You have administrator privileges.

Procedure

  1. List the kernels installed on the machine. 

    # ls -a /boot/vmlinuz-*
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    /boot/vmlinuz-0-rescue-2930657cd0dc43c2b75db480e5e5b4a9
/boot/vmlinuz-4.18.0-330.el8.x86_64
/boot/vmlinuz-4.18.0-330.rt7.111.el8.x86_64
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  2. Add a specific kdump kernel to the system’s Grand Unified Bootloader (GRUB) configuration.

    For example: 

    # grubby --update-kernel=vmlinuz-4.18.0-330.el8.x86_64 --args="crashkernel=xxM"
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    xxM is the required memory reserve in megabytes.

  3. In terms of UKI, add the crashkernel= command-line parameter to a specific UKI by copying an add-on associated with the required argument from /lib/modules/$(uname -r)/vmlinuz-virt.efi.extra.d/ to /boot/efi/EFI/Linux/<machine-id>-<kernel-version>.efi.extra.d/.

  4. Enable the kdump service. 

    # systemctl enable --now kdump.service
    Copy to Clipboard Toggle word wrap

Verification

  • Check that the kdump service is running. 

    # systemctl status kdump.service
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    kdump.service - Crash recovery kernel arming
     Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: disabled)
     Active: active (live)
    Copy to Clipboard Toggle word wrap

17.3. Disabling the kdump service
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You can stop the kdump.service and disable the service from starting on your RHEL 9 systems.

Prerequisites

Procedure

  1. To stop the kdump service in the current session: 

    # systemctl stop kdump.service
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  2. To disable the kdump service: 

    # systemctl disable kdump.service
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Warning

It is recommended to set kptr_restrict=1 as default. When kptr_restrict is set to (1) as default, the kdumpctl service loads the crash kernel regardless of whether the Kernel Address Space Layout (KASLR) is enabled.

If kptr_restrict is not set to 1 and KASLR is enabled, the contents of /proc/kore file are generated as all zeros. The kdumpctl service fails to access the /proc/kcore file and load the crash kernel. The kexec-kdump-howto.txt file displays a warning message, which recommends you to set kptr_restrict=1. Verify for the following in the sysctl.conf file to ensure that kdumpctl service loads the crash kernel:

  • Kernel kptr_restrict=1 in the sysctl.conf file.
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