Add module for zswap

Even though these options are rather opinionated rather than generally
useful, it makes sense to have an option for that because I'm going to
use it for my managed machines as well.

Signed-off-by: aszlig <aszlig@nix.build>

2 files changed, 39 insertions, 0 deletions

diff --git a/modules/module-list.nix b/modules/module-list.nix
index f8a4e299..c75b8dae 100644
--- a/modules/module-list.nix
+++ b/modules/module-list.nix
@@ -14,6 +14,7 @@
   ./system/iso.nix
   ./system/kernel/bfq
   ./system/kernel/rckernel.nix
+  ./system/kernel/zswap.nix
   ./user/aszlig/profiles/base.nix
   ./user/aszlig/profiles/managed.nix
   ./user/aszlig/profiles/workstation
diff --git a/modules/system/kernel/zswap.nix b/modules/system/kernel/zswap.nix
new file mode 100644
index 00000000..43b89ca6
--- /dev/null
+++ b/modules/system/kernel/zswap.nix
@@ -0,0 +1,38 @@
+{ config, lib, ... }:
+
+let
+  kernelVersion = config.boot.kernelPackages.kernel.version;
+  hasZstd = lib.versionAtLeast kernelVersion "4.18";
+in {
+  options.vuizvui.system.kernel.zswap.enable = lib.mkOption {
+    type = lib.types.bool;
+    default = false;
+    description = ''
+      Whether to enable support for zswap with the use of the
+      <literal>z3fold</literal> for pooling and <literal>zstd</literal> for
+      compression, if available (otherwise it falls back to
+      <literal>lzo</literal>).
+
+      Zswap is a compressed cache for swap pages, which is especially useful
+      for machines with limited RAM.
+    '';
+  };
+
+  config = lib.mkIf config.vuizvui.system.kernel.zswap.enable {
+    boot.kernelPatches = lib.singleton {
+      name = "zswap-config";
+      patch = null;
+      extraConfig = ''
+        CRYPTO_${if hasZstd then "ZSTD" else "LZO"} y
+        ZSWAP y
+        Z3FOLD y
+      '';
+    };
+
+    boot.kernelParams = [
+      "zswap.enabled=1"
+      "zswap.zpool=z3fold"
+      "zswap.compressor=${if hasZstd then "zstd" else "lzo"}"
+    ];
+  };
+}