CentOS 7: LVM swap extension not shown by the "free" command - centos7

Question

I'm running a CentOS 7 guest on a VirtualBox 6 on Windows. The result of the free command is as follows:
$ free -h
total used free shared buff/cache available
Mem: 15G 2.4G 11G 162M 1.5G 12G
Swap: 1.2G 0B 1.2G
showing that the swap partition has 1.2 GB. I need to extend it to at least 2GB. So, with the guest stopped, I added a new volume of 1.2 GB and, after having rebooted, I did as follows:
$ sudo pvcreate /dev/sdb
$ sudo vgextend centos /dev/sdb
$ sudo lvextend -L+1G /dev/centos/swap
Now, the lvdisplay command shows the new created volume, as follows:
$ sudo lvdisplay
--- Logical volume ---
LV Path /dev/centos/swap
LV Name swap
VG Name centos
LV UUID 1OT4R8-69eL-vczL-zydM-XrwS-jA47-YfikMS
LV Write Access read/write
LV Creation host, time localhost, 2019-12-30 22:01:35 +0100
LV Status available
# open 2
LV Size <2.20 GiB
Current LE 563
Segments 2
Allocation inherit
Read ahead sectors auto
- currently set to 8192
Block device 253:1
--- Logical volume ---
LV Path /dev/centos/root
LV Name root
VG Name centos
LV UUID hGDGPf-iPMB-TUtM-nqRv-aDNd-D3mw-W15H8Z
LV Write Access read/write
LV Creation host, time localhost, 2019-12-30 22:01:35 +0100
LV Status available
# open 1
LV Size <76.43 GiB
Current LE 19565
Segments 3
Allocation inherit
Read ahead sectors auto
- currently set to 8192
Block device 253:0
The fstab file looks as follows:
dev/mapper/centos-root / xfs defaults 0 0
UUID=4ef0416f-1617-40da-99d2-83896d808eed /boot xfs defaults 0 0
/dev/mapper/centos-swap swap swap defaults 0 0
showing that the swap is allocated on the /dev/mapper/centos-swap partition. Here is the out put of the fstab command:
Disk /dev/mapper/centos-root: 82.1 GB, 82061557760 bytes, 160276480 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk /dev/mapper/centos-swap: 2361 MB, 2361393152 bytes, 4612096 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
But after reboot the swapon command doesn't seem to reflect the extension:
$ sudo swapon -s
Filename Type Size Used Priority
/dev/dm-1 partition 1257468 0 -2
For some reason, the swap doesn't seem to be on the /dev/mapper/centos-swap partition but on /dev/dm-1, which doesn't even exist. And the free command still shows the same result like in the beggining:
$ free -h
total used free shared buff/cache available
Mem: 15G 2.4G 11G 155M 1.5G 12G
Swap: 1.2G 0B 1.2G
and the /proc/swaps:
$ cat /proc/swaps
Filename Type Size Used Priority
/dev/dm-1 partition 1257468 0 -2
What am I missing here ?
Seymour

Answer 1

I'm answering my own question. The issue is simply solved by running the following command:
sudo mkswap /dev/mapper/centos-swap
After that, the free command shows the new increased swap space and the /proc/swaps file also reflect that.
I found the solution by chance, while surfing for another topic. It seems that, as a matter of fact, after having created the physical volume and after having extended the virtual group and the logical volume, it's not enough to declare the new swap with swapon command, but it also requires to effectively "make" the swap using the mkswap command.
Don't ask me why, this is the way it works :-).