how to mount root partition in rw mode?

I clean fstab file! of course I have a backup. So I must restore it! When I try to do it, I encountered a problem. root partition was mounted in read-only mode! but I need to rw mode! how I remount root partition? (my root partition is btrfs)
I try by

mount -t btrfs -o rw, remount /dev/sda2  /

but it does not work!

> blkid

/dev/sda1: partUUID="blablabla1"
/dev/sda2: UUID="blablabla21" UUID_SUB="blablabla22" TYPE="btrfs" PTYPE="dos" PARTUUID="blablabla23"
/dev/sda3: UUID="blablabla31" TYPE="ext4" PARTUUID="blablabla32"
/dev/sda4: UUID="blablabla41" TYPE="ext4" PARTUUID="blablabla42"
/dev/sda5: UUID="blablabla51" TYPE="ext4" PARTUUID="blablabla52"
/dev/sda6: UUID="blablabla61" TYPE="crypto_LUCKS" PARTUUID="blablabla62"
/dev/sda7: UUID="blablabla71" TYPE="swap" PARTLABEL="primary" PARTUUID="blablabla72"

Okay, so the remount does not work, do unmounting and then mounting work? Any error reported?

Also have a look in “dmesg” and “journalctl -b” if you see any error related to /dev/sda2