How to get exact amount of the free memory on linux

I have a Linux server with 128 GB of RAM and several KVM virtual machines and the usage of the memory usage of a server is much more than I expected. All virtual machines are using about 32 gigabytes of memory in total, but when I checking memory usage with the free -m command, it reports memory usage over 80 gigabytes. How to find out which programs use extra memory?

There is no significant shared memory usage.

# ipcs -m --human

------ Shared Memory Segments --------
key        shmid      owner      perms      size       nattch     status      
0x00000000 0          root       644           80B     2                       
0x00000000 32769      root       644           16K     2                       
0x00000000 65538      root       644          280B     2                       
0x00000000 163843     zabbix     600          576B     6          dest         
0x00000000 196612     zabbix     600          1.2M     6          dest 

I've tried to sum the memory usage by ps command, it gives me roughly 40 gigabytes.

# ps -o rss ax | perl -nle '$sum += $_ } END { print $sum'
40706900

But free -m reports that memory usage is over 95 gigabytes (used-buff/cache).

# free -m
              total        used        free      shared  buff/cache   available
Mem:         128900      112804        1853        1690       14242       13355
Swap:             0           0           0

On the other hand tmpfs says that there is 63 GB of free memory.

# df -h
Filesystem                  Size  Used Avail Use% Mounted on
...
tmpfs                        63G   57M   63G   1% /dev/shm
...

Is there some way to find out how much free memory do I have?

UPD: Found a bunch of answers, unreclaimable slabs (50GB), and ZFS ARC cache (6GB). Both memory drains are well undocumented.

Slabs: https://serverfault.com/questions/877257/why-cant-my-linux-kernel-reclaim-its-slab-memory

ARC cache: https://askubuntu.com/questions/836246/general-way-to-monitor-all-memory-usages