Free swap file space contained on root partition without rebooting to free space checked by df

I have created a temporary Swap file (/var/cache/swap/myswap) of 500GB on a Ubuntu worstation at work wiht 1TB RAM. This swapfile is on a BTRFS filesystem.

I don't need anymore of this swapfile now and I would like that space taken by this swapfile doesn't appear anymore when I do a "df" command.

I would like to free this space without rebooting.

I tried : $ swapoff -a

I have also tried : $ swapoff /var/cache/swap/myswap

I have removed too basically the file /var/cache/swap/myswap

But still space 500GB are taken into account when I do a df command (I have still 64% root partition "/" instead of 10% when there is not swapfile of 500GB)

How could I make really this swapfile to not be taken into account with df command without having to reboot ? I mean, that Swapfile removing to be effective.