add btrfs swap support

example/btrfs-subvolumes.nix

@@ -45,6 +45,25 @@
                   };
                   # This subvolume will be created but not mounted
                   "/test" = { };
+                  # Subvolume for the swapfile
+                  "/swap" = {
+                    mountpoint = "/.swapvol";
+                    swap = {
+                      swapfile.size = "20M";
+                      swapfile2.size = "20M";
+                      swapfile2.path = "rel-path";
+                    };
+                  };
+                };
+
+                mountpoint = "/partition-root";
+                swap = {
+                  swapfile = {
+                    size = "20M";
+                  };
+                  swapfile1 = {
+                    size = "20M";
+                  };
                 };
               };
             };

example/luks-btrfs-subvolumes.nix

@@ -48,6 +48,10 @@
                       mountpoint = "/nix";
                       mountOptions = [ "compress=zstd" "noatime" ];
                     };
+                    "/swap" = {
+                      mountpoint = "/.swapvol";
+                      swap.swapfile.size = "20M";
+                    };
                   };
                 };
               };

lib/default.nix

@@ -192,7 +192,7 @@ let
           || lib.hasSuffix "Hook" n
           || isAttrsOfSubmodule o
           # TODO don't hardcode diskoLib.subType options.
-          || n == "content" || n == "partitions" || n == "datasets"
+          || n == "content" || n == "partitions" || n == "datasets" || n == "swap"
         );
       in
       lib.toShellVars

lib/types/btrfs.nix

@@ -1,4 +1,40 @@
 { config, options, diskoLib, lib, rootMountPoint, parent, device, ... }:
+let
+  swapType = lib.mkOption {
+    type = lib.types.attrsOf (lib.types.submodule ({ name, ... }: {
+      options = {
+        size = lib.mkOption {
+          type = lib.types.strMatching "^([0-9]+[KMGTP])?$";
+          description = "Size of the swap file (e.g. 2G)";
+        };
+
+        path = lib.mkOption {
+          type = lib.types.str;
+          default = name;
+          description = "Path to the swap file (relative to the mountpoint)";
+        };
+      };
+    }));
+    default = { };
+    description = "Swap files";
+  };
+
+  swapConfig = { mountpoint, swap }:
+    {
+      swapDevices = builtins.map
+        (file: {
+          device = "${mountpoint}/${file.path}";
+        })
+        (lib.attrValues swap);
+    };
+
+  swapCreate = mountpoint: swap:
+    lib.concatMapStringsSep
+      "\n"
+      (file: ''btrfs filesystem mkswapfile --size ${file.size} "${mountpoint}/${file.path}"'')
+      (lib.attrValues swap);
+
+in
 {
   options = {
     type = lib.mkOption {
@@ -50,6 +86,7 @@
             default = null;
             description = "Location to mount the subvolume to.";
           };
+          swap = swapType;
         };
       }));
       default = { };
@@ -60,6 +97,7 @@
       default = null;
       description = "A path to mount the BTRFS filesystem to.";
     };
+    swap = swapType;
     _parent = lib.mkOption {
       internal = true;
       default = parent;
@@ -75,12 +113,21 @@
       inherit config options;
       default = ''
         mkfs.btrfs ${config.device} ${toString config.extraArgs}
+        ${lib.optionalString (config.swap != {}) ''
+          (
+            MNTPOINT=$(mktemp -d)
+            mount ${device} "$MNTPOINT" -o subvol=/
+            trap 'umount $MNTPOINT; rm -rf $MNTPOINT' EXIT
+            ${swapCreate "$MNTPOINT" config.swap}
+          )
+        ''}
         ${lib.concatMapStrings (subvol: ''
           (
             MNTPOINT=$(mktemp -d)
             mount ${config.device} "$MNTPOINT" -o subvol=/
             trap 'umount $MNTPOINT; rm -rf $MNTPOINT' EXIT
             btrfs subvolume create "$MNTPOINT"/${subvol.name} ${toString subvol.extraArgs}
+            ${swapCreate "$MNTPOINT/${subvol.name}" subvol.swap}
           )
         '') (lib.attrValues config.subvolumes)}
       '';
@@ -141,6 +188,14 @@
             options = config.mountOptions;
           };
         })
+        (map
+          (subvol: swapConfig {
+            inherit (subvol) mountpoint swap;
+          })
+          (lib.attrValues config.subvolumes))
+        (swapConfig {
+          inherit (config) mountpoint swap;
+        })
       ];
       description = "NixOS configuration";
     };

tests/btrfs-subvolumes.nix

@@ -11,6 +11,10 @@ diskoLib.testLib.makeDiskoTest {
     machine.succeed("btrfs subvolume list / | grep -qs 'path test$'");
     machine.succeed("btrfs subvolume list / | grep -qs 'path nix$'");
     machine.succeed("btrfs subvolume list / | grep -qs 'path home$'");
+    machine.succeed("test -e /.swapvol/swapfile");
+    machine.succeed("test -e /.swapvol/rel-path");
+    machine.succeed("test -e /partition-root/swapfile");
+    machine.succeed("test -e /partition-root/swapfile1");
   '';
 }
 

tests/luks-btrfs-subvolumes.nix

@@ -9,5 +9,6 @@ diskoLib.testLib.makeDiskoTest {
     machine.succeed("cryptsetup isLuks /dev/vda2");
     machine.succeed("btrfs subvolume list / | grep -qs 'path nix$'");
     machine.succeed("btrfs subvolume list / | grep -qs 'path home$'");
+    machine.succeed("test -e /.swapvol/swapfile");
   '';
 }