4.4 kitkat - Can't mount /system as read/write, even though I'm rooted

My phone is rooted, and I tried to remount the system partition in a terminal emulator like so:

mount -o remount,rw -t yaffs2 /dev/block/mtdblock3 /system
mount -o rw,remount /system

Both commands gave the output:

mount : read-only file system

Because of that I can't install Busybox, Xposed, etc. I'm using Android 4.4.2.