fstab 添加 swap file 无效？

fstab 添加了 swapfile，但是开机不自动使用，mount -a 也无效，而且没有任何错误提示。 swapon 可以正常启用 sqapfile。

请问这是什么原因照成的？

root@jd00000:~# cat /etc/fstab 
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda1 during installation
UUID=53030b5c-5cb7-490d-9a5b-767b10cafc02 /               ext4    errors=remount-ro 0       1
/var/swapfile	none	swap	sw	0	0

root@jd00000:~# ls -al /var/swapfile 
-rw------- 1 root root 4294967296 Nov 13 15:17 /var/swapfile
root@jdu4e00u53f7:~# mkswap /var/swapfile 
mkswap: /var/swapfile: warning: wiping old swap signature.
Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
no label, UUID=3731d030-49f9-47ee-a9c1-eb420497d65a
root@jd00000:~# mount -a
root@jd000007:~# free -h
              total        used        free      shared  buff/cache   available
Mem:           2.0G        1.8G         70M         15M         70M         13M
Swap:            0B          0B          0B


root@jdu00000:~# cat /etc/fstab 
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda1 during installation
UUID=53030b5c-5cb7-490d-9a5b-767b10cafc02 /               ext4    errors=remount-ro 0       1
/var/swapfile swap swap defaults 0 0

root@jd00000:~# vi /etc/fstab 
root@jd00000:~# mount -a
root@jdu0000:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            981M     0  981M   0% /dev
tmpfs           201M  5.6M  195M   3% /run
/dev/vda1        30G   12G   17G  40% /
tmpfs          1001M   16K 1001M   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs          1001M     0 1001M   0% /sys/fs/cgroup
overlay          30G   12G   17G  40% /var/lib/docker/overlay2/42273be454d07f9a62d4807d5203d55e143e693b7d215990bf55932acaa636a3/merged
shm              64M  4.0K   64M   1% /var/lib/docker/containers/8af0a8d68cd4d5b5845c9c53c2dcd9a3d7aed18b5e6c04438146d2d682185cf1/shm
tmpfs           201M     0  201M   0% /run/user/0
root@jd00000:~# free -h
              total        used        free      shared  buff/cache   available
Mem:           2.0G        1.8G         71M         15M         61M         10M
Swap:            0B          0B          0B