ops/nixos: globally enable zramSwap

ops/nixos/lib/common.nix

@@ -45,6 +45,11 @@ in
 
     time.timeZone = mkDefault "Etc/UTC";
 
+    zramSwap = {
+      enable = true;
+      memoryMax = 4 * 1024 * 1024 * 1024;
+    };
+
     environment.systemPackages = with pkgs; [
       vim rxvt_unicode.terminfo tmux rebuilder tailscale rsync
       (mercurial.overridePythonAttrs (origAttrs: {

third_party/nixpkgs/nixos/modules/config/zram.nix

@@ -80,6 +80,15 @@ in
         '';
       };
 
+      memoryMax = mkOption {
+        default = null;
+        type = with types; nullOr int;
+        description = ''
+          Maximum total amount of memory (in bytes) that can be used by the zram
+          swap devices.
+        '';
+      };
+
       priority = mkOption {
         default = 5;
         type = types.int;
@@ -146,7 +155,12 @@ in
 
               # Calculate memory to use for zram
               mem=$(${pkgs.gawk}/bin/awk '/MemTotal: / {
-                  print int($2*${toString cfg.memoryPercent}/100.0/${toString devicesCount}*1024)
+                  value=int($2*${toString cfg.memoryPercent}/100.0/${toString devicesCount}*1024);
+                    ${if cfg.memoryMax != null then ''
+                      memory_max=int(${toString cfg.memoryMax}/${toString devicesCount});
+                      if (value > memory_max) { value = memory_max }
+                    '' else ""}
+                  print value
               }' /proc/meminfo)
 
               ${pkgs.utillinux}/sbin/zramctl --size $mem --algorithm ${cfg.algorithm} /dev/${dev}