USB drive not extending, mtab isn't looking good

I have noticed my 4GB USB drive cannot extend to overlayfs:/overlay, the mtab isn't correctly set up, how do I solve the problem?

root@OpenWrt:~# grep -e /overlay /etc/mtab
**/dev/ubi0_1 /overlay ubifs rw,noatime,assert=read-only,ubi=0,vol=1 0 0**
overlayfs:/overlay / overlay rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work 0 0
root@OpenWrt:~# 

root@OpenWrt:~# df -h /overlay /
Filesystem                Size      Used Available Use% Mounted on
/dev/ubi0_1              17.1M      7.2M      9.0M  44% /overlay
overlayfs:/overlay       17.1M      7.2M      9.0M  44% /
root@OpenWrt:~#

[https://openwrt.org/docs/guide-user/additional-software/extroot_configuration](https://openwrt.org/docs/guide-user/additional-software/extroot_configuration)
# grep -e /overlay /etc/mtab
/dev/sda1 /overlay ext4 rw,relatime,data=ordered
overlayfs:/overlay / overlay rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work