sysconfig: use initramfs_options to reduce memory usage

[licliu]

Starting with Linux 6.18, Fedora 42, RHEL 10.2, and RHEL 9.8, the kernel supports the initramfs_options boot parameter.

Sets initramfs_options=size=90% can increase the size of the tmpfs used for initramfs decompression from the default 50% to 90% of memory.

This change greatly reduces memory pressure during decompression, allowing for a reduction in the memory reserved for the crashkernel.

[gemini-code-assist]

Summary of Changes

Hello @licliu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request optimizes system memory usage during the boot process by leveraging the new initramfs_options kernel parameter. By increasing the temporary filesystem size for initramfs decompression, it alleviates memory pressure, ultimately contributing to more efficient memory allocation for critical system components like the crashkernel.

Highlights

• New Kernel Boot Parameter: Introduces the initramfs_options boot parameter, supported by Linux 6.18 and newer distributions like Fedora 42, RHEL 10.2, and RHEL 9.8.
• Memory Optimization: Sets initramfs_options=size=90% across all architectures in the KDUMP_COMMANDLINE_APPEND array, increasing the tmpfs size for initramfs decompression from the default 50% to 90% of memory.
• Reduced Memory Pressure: This change significantly reduces memory pressure during initramfs decompression, which in turn allows for a potential reduction in the memory reserved for the crashkernel.

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[gemini-code-assist]

Code Review

This pull request adds the initramfs_options=size=90% kernel parameter to reduce memory pressure during kdump initramfs decompression. This is a good improvement for systems with newer kernels.

However, there are a couple of issues to consider:

1. Incorrect Memory Estimation: The do_estimate function in kdumpctl calculates the recommended crashkernel memory. It contains logic based on the default 50% tmpfs size for initramfs. With your change to 90%, this estimation is no longer accurate and will recommend a much larger memory size than necessary, which counteracts the goal of this PR. The calculation in kdumpctl (line 1610) should be updated to reflect the new tmpfs size. Based on the existing formulas in the code, the multiplier for initrd_size should probably change from 3 to approximately 2.11 (which is 1 + 1/0.9).

2. Unconditional Parameter: The initramfs_options parameter is added unconditionally. According to the PR description, this is only supported on kernels 6.18 and newer. While older kernels will likely ignore this parameter, it would be cleaner to add it conditionally in kdumpctl based on the kernel version. This would also allow the do_estimate function to adjust its calculation based on whether the option is actually being used.

Addressing these points would make the change more robust and fully achieve the goal of optimizing memory usage.

[gemini-code-assist]

The option initramfs_options=size=90% is repeated for every architecture. This creates code duplication and makes future maintenance harder. If this value needs to be changed, it will have to be updated in 7 different places, which is error-prone.

To improve maintainability, you could define this option in a variable and append it programmatically to all entries. For example, you could add a loop after the array definition:

INITRAMFS_OPTS=" initramfs_options=size=90%"
for arch in "${!KDUMP_COMMANDLINE_APPEND[@]}"; do
    KDUMP_COMMANDLINE_APPEND[$arch]+=$INITRAMFS_OPTS
done

This would centralize the option and make the code cleaner and easier to manage.

[licliu]

Good idea.

[coiby]

Hi @licliu, to further extend Gemini's idea, how about extracting all values shared by all architectures (something like this commit)? If you think it's a good idea, I will create a PR and address this feedback in that PR.

[licliu]

Hi @coiby, I think that's great.

[licliu]

The older kernels will ignore this option so I think it will not be a big problem.
For do_estimate(), I plan to change it next version.