Unaccounted used memory after JVM shutdown

Question

After shutting down a Java application (that does heavy usage of DirectByteBuffer) running inside a Docker Container, there is a lot of unaccounted used memory:

$ free -hg
               total        used        free      shared  buff/cache   available
 Mem:           755G        305G        449G         17M        448M        448G
 Swap:          4.0G          0B        4.0G

Doing something like this:

$ ps -e -o pid= -o comm= -o rss= | awk 'BEGIN{rss_total = 0} {rss_total = rss_total + $3} END {print "RSS total(GB): " rss_total/1024/1024}'
RSS total(GB): 0.698826

I am missing around 300GB(!) of memory.

I am aware that the total sum of RSS won't match the numbers reported by free (due to share pages, buffers, caches, etc) but still...300GB?

Contents of /proc/meminfo folow:

MemTotal:       792419424 kB
MemFree:        471276328 kB
MemAvailable:   470340492 kB
Buffers:              52 kB
Cached:           272672 kB
SwapCached:            0 kB
Active:           477128 kB
Inactive:         170892 kB
Active(anon):     365132 kB
Inactive(anon):    27764 kB
Active(file):     111996 kB
Inactive(file):   143128 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       4194300 kB
SwapFree:        4194300 kB
Dirty:                 8 kB
Writeback:             0 kB
AnonPages:        375316 kB
Mapped:           216800 kB
Shmem:             17596 kB
Slab:             191792 kB
SReclaimable:      58096 kB
SUnreclaim:       133696 kB
KernelStack:       14512 kB
PageTables:        13808 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    400404012 kB
Committed_AS:    2241808 kB
VmallocTotal:   34359738367 kB
VmallocUsed:     1787248 kB
VmallocChunk:   33821919228 kB
HardwareCorrupted:     0 kB
AnonHugePages:         0 kB
CmaTotal:              0 kB
CmaFree:               0 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:      450496 kB
DirectMap2M:    804855808 kB

The application utilizes Active Pivot an in-memory database.

The server runs with vm.overcommit_memory = 1 this a requirement by the Active Pivot component.

The container image is build on top of anapsix / alpine-java : 8u192b12_server-jre