[RFC] mm: why active file + inactive file is larger than cached + buffers?

[Xishi Qiu <qiuxishi@huawei.com>]

Hi, I find the active file + inactive file is larger than cached + buffers, about 5G,
and can not free it by "echo 3 > /proc/sys/vm/drop_caches"

The meminfo shows that the mapped is also very small, so maybe some get the page? (e.g. get_user_pages())
Then it will dec the count of NR_FILE_PAGES when delete from page cache, but because of the count,
the page can not delete from lru, so left active file + inactive file so large, right?

MemTotal:       61528660 kB
MemFree:        48678936 kB
MemAvailable:   53657348 kB
Buffers:           10832 kB
Cached:           640340 kB
SwapCached:            0 kB
Active:          5933664 kB
Inactive:         588968 kB
Active(anon):    1022200 kB
Inactive(anon):   533248 kB
Active(file):    4911464 kB
Inactive(file):    55720 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:             10952 kB
Writeback:             0 kB
AnonPages:        980572 kB
Mapped:            85464 kB
Shmem:            581580 kB
Slab:             343480 kB
SReclaimable:     264632 kB
SUnreclaim:        78848 kB
KernelStack:        8368 kB
PageTables:        18012 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    28667176 kB
Committed_AS:    4144064 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      265848 kB
VmallocChunk:   34359417096 kB
HardwareCorrupted:     0 kB
AnonHugePages:    688128 kB
HugePages_Total:       4
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:    1048576 kB
DirectMap4k:      182112 kB
DirectMap2M:    11294720 kB
DirectMap1G:    51380224 kB