[RFC] why the amount of cache from "free -m" and /proc/meminfo are different?

[Xishi Qiu]

[root@localhost ~]# free -m
              total        used        free      shared  buff/cache   available
Mem:          48295         574       41658           8        6062       46344
Swap:         24191           0       24191

[root@localhost ~]# cat /proc/meminfo
MemTotal:       49454896 kB
MemFree:        42658360 kB
MemAvailable:   47456052 kB
Buffers:               0 kB
Cached:          3727824 kB
SwapCached:            0 kB
Active:          2010196 kB
Inactive:        1784204 kB
Active(anon):      66612 kB
Inactive(anon):     8948 kB
Active(file):    1943584 kB
Inactive(file):  1775256 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:      24772604 kB
SwapFree:       24772604 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         66672 kB
Mapped:            38440 kB
Shmem:              8984 kB
Slab:            2480092 kB
SReclaimable:    1291500 kB
SUnreclaim:      1188592 kB
KernelStack:        4480 kB
PageTables:         4928 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    49500052 kB
Committed_AS:     254220 kB
VmallocTotal:   34359738367 kB
VmallocUsed:           0 kB
VmallocChunk:          0 kB
HardwareCorrupted:     0 kB
AnonHugePages:     18432 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:      140864 kB
DirectMap2M:     5093376 kB
DirectMap1G:    47185920 kB

[root@localhost ~]# uname -a
Linux localhost.localdomain 4.5.0-rc1-0.1-default+ #4 SMP Sat Jan 30 05:38:10 EST 2016 x86_64 x86_64 x86_64 GNU/Linux

[root@localhost ~]# rpm -qa | grep procps
procps-ng-3.3.10-3.el7.x86_64