nixos/swap: add discardPolicy option

Add option for activating discards on swap partitions (none, once, pages and both).

nixos/modules/config/swap.nix

@@ -114,6 +114,19 @@ let
         '';
       };
 
+      discardPolicy = mkOption {
+        default = null;
+        example = "once";
+        type = types.nullOr (types.enum ["once" "pages" "both" ]);
+        description = ''
+          Specify the discard policy for the swap device. If "once", then the
+          whole swap space is discarded at swapon invocation. If "pages",
+          asynchronous discard on freed pages is performed, before returning to
+          the available pages pool. With "both", both policies are activated.
+          See swapon(8) for more information.
+        '';
+      };
+
       deviceName = mkOption {
         type = types.str;
         internal = true;

nixos/modules/tasks/filesystems.nix

@@ -238,8 +238,11 @@ in
         skipCheck = fs: fs.noCheck || fs.device == "none" || builtins.elem fs.fsType fsToSkipCheck;
         # https://wiki.archlinux.org/index.php/fstab#Filepath_spaces
         escape = string: builtins.replaceStrings [ " " "\t" ] [ "\\040" "\\011" ] string;
-        swapOptions = sw: "defaults"
-          + optionalString (sw.priority != null) ",pri=${toString sw.priority}";
+        swapOptions = sw: concatStringsSep "," (
+          [ "defaults" ]
+          ++ optional (sw.priority != null) "pri=${toString sw.priority}"
+          ++ optional (sw.discardPolicy != null) "discard${optionalString (sw.discardPolicy != "both") "=${toString sw.discardPolicy}"}"
+        );
       in ''
         # This is a generated file.  Do not edit!
         #