how to get total and used memmory using special files in filesystem in linux?

how using /sys/fs to get total memmory free and used size?
I know total memory of the process

cat /proc/$pid/smaps | grep -i pss |  awk '{Total+=$2} END {print Total/1024/1024" GB"}'

the file /sys/fs/cgroup/memory.stat contains a lot of information