the result of ngx_shm and smap is different ?

[zeech3]

Hi , I use ngx_shm to view my shared_memory , the result is :

shm zone "db_shared_memory"
owner: ngx_http_dns
total size: 102400 KB
free pages: 72560 KB (18140 pages, 1834 blocks)

but the /proc/pid/smap shows the sharememory is

7fdf592b1000-7fdf5f6b1000 rw-s 00000000 00:05 248922457 /dev/zero (deleted)
Size: 102400 kB
Rss: 42044 kB
Pss: 10527 kB
Shared_Clean: 0 kB
Shared_Dirty: 42044 kB
Private_Clean: 0 kB
Private_Dirty: 0 kB
Referenced: 41444 kB
Anonymous: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Locked: 0 kB
VmFlags: rd wr sh mr mw me ms sd

why Shared_Dirty != total size - free page???