Document and uppercase vars

playbooks/roles/swapfile/defaults/main.yml

 ---
-# To enable the swapfile role, set this variable using a MB or GB suffix
-# e.g. SWAPFILE_SIZE: 512MB
-# To disable the role, leave the default value of 'false'
-SWAPFILE_SIZE: false
+# Size of the desired swap file. Use a MB or GB suffix.
+SWAPFILE_SIZE: 2GB
+# SWAPFILE_LOCATION: Location of the swap file managed by this role.
+SWAPFILE_LOCATION: /swapfile
+# SWAPFILE_USE_DD: set this to True if your filesystem does not support 
+# fallocate (e.g. if you use ext3). dd will then be used instead of fallocate.
+SWAPFILE_USE_DD: False
 
+#
+# Advanced, optional settings: 
+#
 
-swapfile_location: /swapfile
-swapfile_swappiness: False
-swapfile_vfs_cache_pressure: False
-swapfile_use_dd: False
+# SWAPFILE_SWAPPINESS: Update sysctl.conf to set the swappiness percentage
+# (vm.swappiness) -- the lower it is, the less your system swaps memory pages.
+# If this is False (default), no change will be made.
+SWAPFILE_SWAPPINESS: False
+# SWAPFILE_VFS_CACHE_PRESSURE: Update sysctl.conf to set the VFS cache pressure.
+# "this percentage value controls the tendency of the kernel to reclaim the
+#  memory which is used for caching of directory and inode objects."
+# If this is False (default), no change will be made.
+SWAPFILE_VFS_CACHE_PRESSURE: False

playbooks/roles/swapfile/tasks/main.yml

 ---
 - name: Write swapfile
   command: |
-    {% if swapfile_use_dd %}
-    dd if=/dev/zero of={{ swapfile_location }} bs=1M count={{ SWAPFILE_SIZE }} creates={{ swapfile_location }}
+    {% if SWAPFILE_USE_DD %}
+    dd if=/dev/zero of={{ SWAPFILE_LOCATION }} bs=1M count={{ SWAPFILE_SIZE }} creates={{ SWAPFILE_LOCATION }}
     {% else %}
-    fallocate -l {{ SWAPFILE_SIZE }} {{ swapfile_location }} creates={{ swapfile_location }}
+    fallocate -l {{ SWAPFILE_SIZE }} {{ SWAPFILE_LOCATION }} creates={{ SWAPFILE_LOCATION }}
     {% endif %}
   register: write_swapfile
   when: SWAPFILE_SIZE != false
 
 - name: Set swapfile permissions
-  file: path={{ swapfile_location }} mode=600
+  file: path={{ SWAPFILE_LOCATION }} mode=600
   when: SWAPFILE_SIZE != false
 
 - name: Create swapfile
-  command: mkswap {{ swapfile_location }}
+  command: mkswap {{ SWAPFILE_LOCATION }}
   register: create_swapfile
   when: SWAPFILE_SIZE != false and write_swapfile.changed
 
 - name: Enable swapfile
-  command: swapon {{ swapfile_location }}
+  command: swapon {{ SWAPFILE_LOCATION }}
   when: SWAPFILE_SIZE != false and create_swapfile.changed
 
 - name: Add swapfile to /etc/fstab
-  lineinfile: dest=/etc/fstab line="{{ swapfile_location }}   none    swap    sw    0   0" state=present
+  lineinfile: dest=/etc/fstab line="{{ SWAPFILE_LOCATION }}   none    swap    sw    0   0" state=present
   when: SWAPFILE_SIZE != false
 
 - name: Configure vm.swappiness
-  lineinfile: dest=/etc/sysctl.conf line="vm.swappiness = {{ swapfile_swappiness }}" regexp="^vm.swappiness[\s]?=" state=present
+  lineinfile: dest=/etc/sysctl.conf line="vm.swappiness = {{ SWAPFILE_SWAPPINESS }}" regexp="^vm.swappiness[\s]?=" state=present
   notify: reload sysctl
-  when: swapfile_swappiness != false
+  when: SWAPFILE_SWAPPINESS != false
 
 - name: Configure vm.vfs_cache_pressure
-  lineinfile: dest=/etc/sysctl.conf line="vm.vfs_cache_pressure = {{ swapfile_vfs_cache_pressure }}" regexp="^vm.vfs_cache_pressure[\s]?=" state=present
+  lineinfile: dest=/etc/sysctl.conf line="vm.vfs_cache_pressure = {{ SWAPFILE_VFS_CACHE_PRESSURE }}" regexp="^vm.vfs_cache_pressure[\s]?=" state=present
   notify: reload sysctl
-  when: swapfile_vfs_cache_pressure != false
+  when: SWAPFILE_VFS_CACHE_PRESSURE != false