diff --git a/nixos/modules/system/boot/stage-2-init.sh b/nixos/modules/system/boot/stage-2-init.sh
index 173453a17f7..330e2f97c5b 100644
--- a/nixos/modules/system/boot/stage-2-init.sh
+++ b/nixos/modules/system/boot/stage-2-init.sh
@@ -85,8 +85,10 @@ done
 
 
 # More special file systems, initialise required directories.
-mkdir -m 0755 /dev/shm
-mount -t tmpfs -o "rw,nosuid,nodev,size=@devShmSize@" tmpfs /dev/shm
+if ! mountpoint -q /dev/shm; then
+    mkdir -m 0755 /dev/shm
+    mount -t tmpfs -o "rw,nosuid,nodev,size=@devShmSize@" tmpfs /dev/shm
+fi
 mkdir -m 0755 -p /dev/pts
 [ -e /proc/bus/usb ] && mount -t usbfs usbfs /proc/bus/usb # UML doesn't have USB by default
 mkdir -m 01777 -p /tmp