On Tue, Dec 28, 2021 at 08:49:30AM +0800, Coiby Xu wrote:

Update crashkernel-howto since crashkernel.default has been removed. The documentation is also simplified as a result.

Signed-off-by: Coiby Xu coxu@redhat.com

crashkernel-howto.txt | 123 +++++++----------------------------------- 1 file changed, 19 insertions(+), 104 deletions(-)

diff --git a/crashkernel-howto.txt b/crashkernel-howto.txt index 20f50e03..15768ab4 100644 --- a/crashkernel-howto.txt +++ b/crashkernel-howto.txt @@ -13,13 +13,14 @@ kdump after you updated the `crashkernel=` value or changed the dump target. Default crashkernel value =========================

-Latest kernel packages include a `crashkernel.default` file installed in kernel -modules folder, available as: +Latest kexec-tools provides "kdumpctl get-default-crashkernel" to retrieve +the default crashkernel value,

• /usr/lib/modules/<kernel>/crashkernel.default

• $ echo $(kdumpctl get-default-crashkernel)
• 1G-4G:192M,4G-64G:256M,64G-102400T:512M

^^^ A

-The content of the file will be taken as the default value of 'crashkernel=', or -take this file as a reference for setting crashkernel value manually. +It will be taken as the default value of 'crashkernel=', you can use +this value as a reference for setting crashkernel value manually.

New installed system @@ -27,7 +28,7 @@ New installed system

Anaconda is the OS installer which sets all the kernel boot cmdline on a newly installed system. If kdump is enabled during Anaconda installation, Anaconda -will use the `crashkernel.default` file as the default `crashkernel=` value on +will use the default crashkernel value as the default `crashkernel=` value on the newly installed system.

Users can override the value during Anaconda installation manually. @@ -36,20 +37,11 @@ Users can override the value during Anaconda installation manually. Auto update of crashkernel boot parameter =========================================

-Following context in this section assumes all kernel packages have a -`crashkernel.default` file bundled, which is true for the latest official kernel -packages. For kexec-tools behavior with a kernel that doesn't have a -`crashkernel.default` file, please refer to the “Custom Kernel” section of this -doc.

-When `crashkernel=` is using the default value, kexec-tools will need to update -the `crashkernel=` value of new installed kernels, since the default value may -change in new kernel packages.

-kexec-tools does so by adding a kernel installation hook, which gets triggered -every time a new kernel is installed, so kexec-tools can do necessary checks and -updates.

+A new release of kexec-tools could update the the default crashkernel value. +By default, kexec-tools would reset crashkernel to the new default value if it +detects old default crashkernel value is used by installed kernels. If you don't +want kexec-tools to update the old default crashkernel to the new default +crashkernel, you can change auto_reset_crashkernel to no in kdump.conf.

Supported Bootloaders

@@ -59,92 +51,13 @@ on `grubby`. If other boot loaders are used, the user will have to update the `crashkernel=` value manually.

-Updating kernel package

-When a new version of package kernel is released in the official repository, the -package will always come with a `crashkernel.default` file bundled. Kexec-tools -will act with following rules:

-If current boot kernel is using the default `crashkernel=` boot param value from -its `crashkernel.default` file, then kexec-tools will update new installed -kernel’s `crashkernel=` boot param using the value from the new installed -kernel’s `crashkernel.default` file. This ensures `crashkernel=` is always using -the latest default value.

-If current boot kernel's `crashkernel=` value is set to a non-default value, the -new installed kernel simply inherits this value.

-On systems using GRUB2 as the bootloader, each kernel has its own boot entry, -making it possible to set different `crashkernel=` boot param values for -different kernels. So kexec-tools won’t touch any already installed kernel's -boot param, only new installed kernel's `crashkernel=` boot param value will be -updated.

-But some utilities like `grub2-mkconfig` and `grubby` can override all boot -entry's boot params with the boot params value from the GRUB config file -`/etc/defaults/grub`, so kexec-tools will also update the GRUB config file in -case old `crashkernel=` value overrides new installed kernel’s boot param.

-Downgrading kernel package

-When upgrading a kernel package, kexec-tools may update the `crashkernel=` value -in GRUB2 config file to the new value. So when downgrading the kernel package, -kexec-tools will also try to revert that update by setting GRUB2 config file’s -`crashkernel=` value back to the default value in the older kernel package. This -will only occur when the GRUB2 config file is using the default `crashkernel=` -value.

-Custom kernel

-To make auto crashkernel update more robust, kexec-tools will try to keep -tracking the default 'crashkernel=` value with kernels that don’t have a -`crashkernel.default` file, such kernels are referred to as “custom kernel” in -this doc. This is only a best-effort support to make it easier debugging and -testing the system.

-When installing a custom kernel that doesn’t have a `crashkernel.default` file, -the `crashkernel=` value will be simply inherited from the current boot kernel.

-When installing a new official kernel package and current boot kernel is a -custom kernel, since the boot kernel doesn’t have a `crashkernel.default` file, -kexec-tools will iterate installed kernels and check if the boot kernel -inherited the default value from any other existing kernels’ -`crashkernel.default` file. If a matching `crashkernel.default` file is found, -kexec-tools will update the new installed kernel `crashkernel=` boot param using -the value from the new installed kernel’s `crashkernel.default` file, ensures -the auto crashkernel value update won’t break over one or two custom kernel -installations.

-It is possible that the auto crashkernel value update will fail when custom -kernels are used. One example is a custom kernel inheriting the default -`crashkernel=` value from an older official kernel package, but later that -kernel package is uninstalled. So when booted with the custom kernel, -kexec-tools can't determine if the boot kernel is inheriting a default -`crashkernel=` value from any official build. In such a case, please refer to -the "Reset crashkernel to default value" section of this doc.

Reset crashkernel to default value

kexec-tools only perform the auto update of crashkernel value when it can confirm the boot kernel's crashkernel value is using its corresponding default -value or inherited from any installed kernel.

-kexec-tools may fail to determine if the boot kernel is using default -crashkernel value in some use cases: -- kexec-tools package is absent during a kernel package upgrade, and the new

• kernel package’s `crashkernel.default` value has changed.

-- Custom kernel is used and the kernel it inherits `crashkernel=` value from is

• uninstalled.

-So it's recommended to reset the crashkernel value if users have uninstalled -kexec-tools or using a custom kernel. +value and auto_reset_crashkernel=yes in kdump.conf. In other cases, the user +can reset the crasherknel value by themselves.

Reset using kdumpctl

@@ -152,12 +65,14 @@ Reset using kdumpctl To make it easier to reset the `crashkernel=` kernel cmdline to this default value properly, `kdumpctl` also provides a sub-command:

• `kdumpctl reset-crashkernel [<kernel version>]`

• `kdumpctl reset-crashkernel [--kernel=path_to_kernel] [--reboot]`

This command will read from the `crashkernel.default` file and reset bootloader's kernel cmdline to the default value. It will also update bootloader config if the bootloader has a standalone config file. User will have to reboot -the machine after this command to make it take effect. +the machine after this command to make it take effect if --reboot is not specified. +For ppc64le, an optional "[--fadump=[on|off|nocma]]" can also be specified to toggle +FADump on/off.

Reset manually

@@ -166,7 +81,7 @@ To reset the crashkernel value manually, it's recommended to use utils like `grubby`. A one liner script for resetting `crashkernel=` value of all installed kernels to current boot kernel's crashkernel.default` is:

• grubby --update-kernel ALL --args "$(cat /usr/lib/modules/$(uname -r)/crashkernel.default)"

• grubby --update-kernel ALL --args "$(kdumpctl get-default-crashkernel)"

^^^ B Either A or B should contain "crashkernel="

Else,

Reviewed-by: Pingfan Liu piliu@redhat.com

Estimate crashkernel

-- 2.31.1