nixos/zram-generator: do not restart on switch (#504305)

nixos/modules/services/system/zram-generator.nix

@@ -36,7 +36,18 @@ in
     ];
 
     systemd.packages = [ cfg.package ];
-    systemd.services."systemd-zram-setup@".path = [ pkgs.util-linux ]; # for mkswap
+    systemd.services."systemd-zram-setup@" = {
+      path = [ pkgs.util-linux ]; # for mkswap
+      # Restart on switch does swapoff -> reset -> reconfigure -> swapon.
+      # Besides risking OOM when swap is in use, the reset races with udev
+      # still holding /dev/zramN open after swapoff: the kernel rejects the
+      # reset with EBUSY (disk_openers > 0), the device stays initialized,
+      # and the following ExecStart fails to write comp_algorithm/disksize
+      # (systemd/zram-generator#226). The unit only "changes" due to store
+      # path churn, so keep the device across switches; real config changes
+      # need a reboot or manual restart.
+      restartIfChanged = false;
+    };
 
     environment.etc."systemd/zram-generator.conf".source =
       settingsFormat.generate "zram-generator.conf" cfg.settings;