cat /etc/redhat-release:
Red Hat Enterprise Linux Server release 7.7 (Maipo)


cloud-init -v:
/usr/bin/cloud-init 19.4+37.g0562ca69-1.el7


cloud-init config file:
# ephemeral0 is a device alias for /dev/disk/cloud/azure_resource-part1 (Azure ephemeral resource disk)
mounts:
   - [ ephemeral0, /mnt/resource ]
swap:
   filename: /mnt/resource/swapfile
   size: 2147483648



cloud-init.log:
2020-02-20 00:01:57,972 - helpers.py[DEBUG]: Running config-mounts using lock (<FileLock using file '/var/lib/cloud/instances/8AE39487-C082-6241-968C-B313BB587C20/sem/config_mounts'>)
2020-02-20 00:01:57,973 - cc_mounts.py[DEBUG]: mounts configuration is [['ephemeral0', '/mnt/resource']]
2020-02-20 00:01:57,973 - util.py[DEBUG]: Reading from /etc/fstab (quiet=False)
2020-02-20 00:01:57,973 - util.py[DEBUG]: Read 532 bytes from /etc/fstab
2020-02-20 00:01:57,973 - cc_mounts.py[DEBUG]: Attempting to determine the real name of ephemeral0
2020-02-20 00:01:57,973 - cc_mounts.py[DEBUG]: Mapped metadata name ephemeral0 to /dev/disk/cloud/azure_resource
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: changed ephemeral0 => /dev/disk/cloud/azure_resource-part1
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: Attempting to determine the real name of ephemeral0
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: Mapped metadata name ephemeral0 to /dev/disk/cloud/azure_resource
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: changed default device ephemeral0 => /dev/disk/cloud/azure_resource-part1
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: Not including ephemeral0, already previously included
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: Attempting to determine the real name of swap
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: changed default device swap => None
2020-02-20 00:01:57,974 - cc_mounts.py[DEBUG]: Ignoring nonexistent default named mount swap
2020-02-20 00:01:57,975 - util.py[DEBUG]: Restoring selinux mode for /mnt (recursive=True)
2020-02-20 00:01:57,975 - util.py[DEBUG]: Running command ['sh', '-c', 'rm -f "$1" && umask 0066 && { fallocate -l "${2}M" "$1" || dd if=/dev/zero "of=$1" bs=1M "count=$2"; } && mkswap "$1" || { r=$?; rm -f "$1"; exit $r; }', 'setup_swap', '/mnt/resource/swapfile', '2048'] with allowed return codes [0] (shell=False, capture=True)
2020-02-20 00:01:58,091 - util.py[DEBUG]: creating swap file '/mnt/resource/swapfile' of 2048MB took 0.116 seconds
2020-02-20 00:01:58,091 - util.py[DEBUG]: Reading from /proc/mounts (quiet=False)
2020-02-20 00:01:58,091 - util.py[DEBUG]: Read 2227 bytes from /proc/mounts
2020-02-20 00:01:58,092 - util.py[DEBUG]: Fetched {'configfs': {'mountpoint': '/sys/kernel/config', 'opts': 'rw,relatime', 'fstype': 'configfs'}, 'devpts': {'mountpoint': '/dev/pts', 'opts': 'rw,seclabel,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000', 'fstype': 'devpts'}, 'debugfs': {'mountpoint': '/sys/kernel/debug', 'opts': 'rw,relatime', 'fstype': 'debugfs'}, 'securityfs': {'mountpoint': '/sys/kernel/security', 'opts': 'rw,nosuid,nodev,noexec,relatime', 'fstype': 'securityfs'}, 'sysfs': {'mountpoint': '/sys', 'opts': 'rw,seclabel,nosuid,nodev,noexec,relatime', 'fstype': 'sysfs'}, 'mqueue': {'mountpoint': '/dev/mqueue', 'opts': 'rw,seclabel,relatime', 'fstype': 'mqueue'}, 'pstore': {'mountpoint': '/sys/fs/pstore', 'opts': 'rw,nosuid,nodev,noexec,relatime', 'fstype': 'pstore'}, '/dev/sda1': {'mountpoint': '/boot', 'opts': 'rw,seclabel,relatime,attr2,inode64,noquota', 'fstype': 'xfs'}, 'hugetlbfs': {'mountpoint': '/dev/hugepages', 'opts': 'rw,seclabel,relatime', 'fstype': 'hugetlbfs'}, 'systemd-1': {'mountpoint': '/proc/sys/fs/binfmt_misc', 'opts': 'rw,relatime,fd=25,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=13534', 'fstype': 'autofs'}, 'selinuxfs': {'mountpoint': '/sys/fs/selinux', 'opts': 'rw,relatime', 'fstype': 'selinuxfs'}, 'cgroup': {'mountpoint': '/sys/fs/cgroup/devices', 'opts': 'rw,seclabel,nosuid,nodev,noexec,relatime,devices', 'fstype': 'cgroup'}, 'tmpfs': {'mountpoint': '/sys/fs/cgroup', 'opts': 'ro,seclabel,nosuid,nodev,noexec,mode=755', 'fstype': 'tmpfs'}, 'proc': {'mountpoint': '/proc', 'opts': 'rw,nosuid,nodev,noexec,relatime', 'fstype': 'proc'}, 'devtmpfs': {'mountpoint': '/dev', 'opts': 'rw,seclabel,nosuid,size=1744700k,nr_inodes=436175,mode=755', 'fstype': 'devtmpfs'}, '/dev/sda2': {'mountpoint': '/', 'opts': 'rw,seclabel,relatime,attr2,inode64,noquota', 'fstype': 'xfs'}, 'rootfs': {'mountpoint': '/', 'opts': 'rw', 'fstype': 'rootfs'}} mounts from proc
2020-02-20 00:01:58,092 - util.py[DEBUG]: Writing to /etc/fstab - wb: [644] 601 bytes
2020-02-20 00:01:58,092 - util.py[DEBUG]: Restoring selinux mode for /etc/fstab (recursive=False)
2020-02-20 00:01:58,093 - util.py[DEBUG]: Restoring selinux mode for /etc/fstab (recursive=False)
2020-02-20 00:01:58,093 - cc_mounts.py[DEBUG]: Changes to fstab: ['- /dev/disk/cloud/azure_resource-part1 /mnt auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2', '+ /dev/disk/cloud/azure_resource-part1 /mnt/resource auto defaults,nofail,x-systemd.requires=cloud-init.service,comment=cloudconfig 0 2', '+ /mnt/resource/swapfile none swap sw,comment=cloudconfig 0 0']
2020-02-20 00:01:58,093 - util.py[DEBUG]: Running command ['swapon', '-a'] with allowed return codes [0] (shell=False, capture=True)
2020-02-20 00:01:58,105 - cc_mounts.py[WARNING]: Activate mounts: FAIL:swapon -a
2020-02-20 00:01:58,106 - util.py[WARNING]: Activate mounts: FAIL:swapon -a
2020-02-20 00:01:58,106 - util.py[DEBUG]: Activate mounts: FAIL:swapon -a
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cloudinit/config/cc_mounts.py", line 495, in handle
    util.subp(cmd)
  File "/usr/lib/python2.7/site-packages/cloudinit/util.py", line 2099, in subp
    cmd=args)
ProcessExecutionError: Unexpected error while running command.
Command: ['swapon', '-a']
Exit code: 255
Reason: -
Stdout:
Stderr: swapon: /mnt/resource/swapfile: swapon failed: Invalid argument
2020-02-20 00:01:58,128 - util.py[DEBUG]: Running command ['mount', '-a'] with allowed return codes [0] (shell=False, capture=True)
2020-02-20 00:01:58,224 - cc_mounts.py[DEBUG]: Activate mounts: PASS:mount -a