I took the following meminfo captures.
I don't figure out how the free memory went from 8GB to almost 25GB, when only about 4GB of slab was freed. There was no change of the proccess memory consumption on time the meminfo output was taken.

First meminfo with 8GB free memory:
MemTotal:       66054256 kB
MemFree:         8344960 kB
Buffers:            1120 kB
Cached:         30172312 kB
SwapCached:            0 kB
Active:         10795428 kB
Inactive:        1914512 kB
Active(anon):   10193124 kB
Inactive(anon):  1441288 kB
Active(file):     602304 kB
Inactive(file):   473224 kB
Unevictable:    26348912 kB
Mlocked:        26348960 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:       8886304 kB
Mapped:         26383052 kB
Shmem:          29097904 kB
Slab:            6006384 kB
SReclaimable:    3512404 kB
SUnreclaim:      2493980 kB
KernelStack:       15240 kB
PageTables:        78724 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    33027128 kB
Committed_AS:   44446908 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      426656 kB
VmallocChunk:   34325375716 kB
HardwareCorrupted:     0 kB
AnonHugePages:   7696384 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        6144 kB
DirectMap2M:     2058240 kB
DirectMap1G:    65011712 kB

Second memory capture with almost 25GB free memory:
MemTotal:       66054256 kB
MemFree:        24949116 kB
Buffers:            1120 kB
Cached:         29085016 kB
SwapCached:            0 kB
Active:         10168904 kB
Inactive:        1461156 kB
Active(anon):   10168216 kB
Inactive(anon):  1441956 kB
Active(file):        688 kB
Inactive(file):    19200 kB
Unevictable:    26317328 kB
Mlocked:        26317376 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:       8861224 kB
Mapped:         26351488 kB
Shmem:          29066248 kB
Slab:            1503440 kB
SReclaimable:     232880 kB
SUnreclaim:      1270560 kB
KernelStack:       15256 kB
PageTables:        79664 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    33027128 kB
Committed_AS:   44418280 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      426656 kB
VmallocChunk:   34325375716 kB
HardwareCorrupted:     0 kB
AnonHugePages:   7665664 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        6144 kB
DirectMap2M:     2058240 kB
DirectMap1G:    65011712 kB