fix(nixos): gate zram kernel sysctl tunings behind modules.zram.enable

The zram module was auto-imported on all hosts via base scanPaths, and its
swappiness=180/page-cluster=0 tunings leaked onto hosts that disabled zram
(e.g. idols-ai with a disk swapfile, pushing cold anon pages to disk swap).
Wrap the whole module in modules.zram.enable (default true); idols-ai now
turns off zram and its tunings with a single switch.

hosts/idols-ai/default.nix

@@ -39,7 +39,8 @@ in
   ];
 
   # Zram consumes physical memory for compression, which can cause a deadlock and system hang if the model size approaches the physical memory limit.
-  zramSwap.enable = lib.mkForce false;
+  # Disable the whole module (zram device + its swappiness=180 sysctl tunings); this host uses a disk swapfile instead.
+  modules.zram.enable = false;
 
   services.sunshine.enable = false;
   services.tuned.ppdSettings.main.default = lib.mkForce "performance";

modules/nixos/base/zram.nix

@@ -1,44 +1,68 @@
-{ lib, ... }:
 {
-  # Enable in-memory compressed devices and swap space provided by the zram kernel module.
+  lib,
-  # By enable this, we can store more data in memory instead of fallback to disk-based swap devices directly,
+  config,
-  # and thus improve I/O performance when we have a lot of memory.
+  ...
-  #
+}:
-  #   https://www.kernel.org/doc/Documentation/blockdev/zram.txt
+let
-  zramSwap = {
+  cfg = config.modules.zram;
-    enable = true;
+in
-    # one of "lzo", "lz4", "zstd"
+{
-    algorithm = lib.mkDefault "zstd";
+  options.modules.zram.enable =
-    # Priority of the zram swap devices.
+    lib.mkEnableOption ''
-    # It should be a number higher than the priority of your disk-based swap devices
+      zram in-memory swap device plus kernel sysctl tunings for it.
-    # (so that the system will fill the zram swap devices before falling back to disk swap).
-    priority = lib.mkDefault 100;
-    # Maximum total amount of memory that can be stored in the zram swap devices (as a percentage of your total memory).
-    # Defaults to 1/2 of your total RAM. Run zramctl to check how good memory is compressed.
-    # This doesn’t define how much memory will be used by the zram swap devices.
-    memoryPercent = lib.mkDefault 50;
-  };
 
-  # Optimizing swap on zram
+      The sysctl tunings (swappiness=180 etc.) are designed for in-memory swap
-  boot.kernel.sysctl = {
+      devices only; they are gated behind this switch so that hosts which
-    # vm.swappiness - Controls kernel preference for swapping (range: 0-200, default: 60)
+      disable zram (e.g. using a disk swapfile instead) also revert to the
-    # For in-memory swap devices like zram/zswap, values above 100 are recommended.
+      kernel default swappiness instead of leaking zram tunings onto disk swap.
-    "vm.swappiness" = lib.mkDefault 180;
 
-    # vm.watermark_boost_factor - Controls aggressiveness of memory reclaim (default: 15000)
+      Enabled by default; hosts that want disk-only swap should set it to false.
-    # Setting to 0 disables watermark boost, preventing premature memory reclamation.
+    ''
-    # This allows fuller memory utilization before the kernel starts reclaiming pages.
+    // {
-    "vm.watermark_boost_factor" = lib.mkDefault 0;
+      default = true;
+    };
 
-    # vm.watermark_scale_factor - Controls kswapd wakeup frequency (range: 1-1000, default: 10)
+  config = lib.mkIf cfg.enable {
-    # A higher value triggers background memory reclamation earlier (at 12.5% memory pressure).
+    # Enable in-memory compressed devices and swap space provided by the zram kernel module.
-    # Value 125 means kswapd becomes active when free memory drops below 1/125 of total memory,
+    # By enable this, we can store more data in memory instead of fallback to disk-based swap devices directly,
-    # balancing memory more proactively to prevent sudden swap storms at high swappiness values.
+    # and thus improve I/O performance when we have a lot of memory.
-    "vm.watermark_scale_factor" = lib.mkDefault 125;
+    #
+    #   https://www.kernel.org/doc/Documentation/blockdev/zram.txt
+    zramSwap = {
+      enable = true;
+      # one of "lzo", "lz4", "zstd"
+      algorithm = lib.mkDefault "zstd";
+      # Priority of the zram swap devices.
+      # It should be a number higher than the priority of your disk-based swap devices
+      # (so that the system will fill the zram swap devices before falling back to disk swap).
+      priority = lib.mkDefault 100;
+      # Maximum total amount of memory that can be stored in the zram swap devices (as a percentage of your total memory).
+      # Defaults to 1/2 of your total RAM. Run zramctl to check how good memory is compressed.
+      # This doesn't define how much memory will be used by the zram swap devices.
+      memoryPercent = lib.mkDefault 50;
+    };
 
-    # vm.page-cluster - Controls swap readahead (range: 0-6, default: 3)
+    # Optimizing swap on zram
-    # 0 means read only 1 page (2^0) at a time, disabling readahead.
+    boot.kernel.sysctl = {
-    # For low-latency devices like zram, readahead hurts performance by fetching unnecessary data.
+      # vm.swappiness - Controls kernel preference for swapping (range: 0-200, default: 60)
-    "vm.page-cluster" = lib.mkDefault 0;
+      # For in-memory swap devices like zram/zswap, values above 100 are recommended.
+      "vm.swappiness" = lib.mkDefault 180;
+
+      # vm.watermark_boost_factor - Controls aggressiveness of memory reclaim (default: 15000)
+      # Setting to 0 disables watermark boost, preventing premature memory reclamation.
+      # This allows fuller memory utilization before the kernel starts reclaiming.
+      "vm.watermark_boost_factor" = lib.mkDefault 0;
+
+      # vm.watermark_scale_factor - Controls kswapd wakeup frequency (range: 1-1000, default: 10)
+      # A higher value triggers background memory reclamation earlier (at 12.5% memory pressure).
+      # Value 125 means kswapd becomes active when free memory drops below 1/125 of total memory,
+      # balancing memory more proactively to prevent sudden swap storms at high swappiness values.
+      "vm.watermark_scale_factor" = lib.mkDefault 125;
+
+      # vm.page-cluster - Controls swap readahead (range: 0-6, default: 3)
+      # 0 means read only 1 page (2^0) at a time, disabling readahead.
+      # For low-latency devices like zram, readahead hurts performance by fetching unnecessary data.
+      "vm.page-cluster" = lib.mkDefault 0;
+    };
   };
 }