nixos/zswap: init module

[pigsinablanket]

Provides a NixOS module for enabling and configuring zswap, a compressed cache for swap pages within the kernel.

Closes #119244.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[acid-bong]

And in the end: why making this a systemd script instead of kernel parameters (to start on boot) + activation scripts (to start and stop on rebuild)?

[acid-bong]

Also change addresses to solves/fixes to automatically close the issue (github only recognizes certain words)

[ShamrockLee]

Thank you for addressing #119244.

Considering that we already have boot.kernel.sysctl that provide mapping for each kernel parameters, is it worth the effort to add another module implemented in a similar manner?

[ShamrockLee]

Thank you for addressing #119244.

Considering that we already have boot.kernel.sysctl that provide mapping for each kernel parameters, is it worth the effort to add another module implemented in a similar manner?

I took back what I said about boot.kernel.sysctl. It might not work for zswap. See #119244 (comment) for detail.

[pigsinablanket]

Thank you for addressing #119244.
Considering that we already have boot.kernel.sysctl that provide mapping for each kernel parameters, is it worth the effort to add another module implemented in a similar manner?

I took back what I said about boot.kernel.sysctl. It might not work for zswap. See #119244 (comment) for detail.

What do you think the path forward is? I see a couple options:

• Get zswap enabled by default as other linux distros have done.
• Create a zswap module wrapper around the parameters, which this PR does. This might imply that everything in /sys/module should have a wrapper around it.
• Do nothing and document it on the wiki on how to configure it via boot.kernelParams https://wiki.nixos.org/wiki/Swap#Zswap_swap_cache.
• Somehow make a boot.kernel.sysmodule option to set parameters in /sys/module.

[ShamrockLee]

• Somehow make a boot.kernel.sysmodule option to set parameters in /sys/module.

I second this. Here's a draft PR that implements said module: #402455

[pigsinablanket]

Closing this PR in favor of a sysmodule config option. See any further discussion here #119244