snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table.
It's slow but very precise.
-Table 1-2: Contents of the status files (as of 4.1)
+Table 1-2: Contents of the status files (as of 4.8)
..............................................................................
Field Content
Name filename of the executable
+ Umask file mode creation mask
State state (R is running, S is sleeping, D is sleeping
in an uninterruptible wait, Z is zombie,
T is traced or stopped)
TracerPid PID of process tracing this process (0 if not)
Uid Real, effective, saved set, and file system UIDs
Gid Real, effective, saved set, and file system GIDs
- Umask file mode creation mask
FDSize number of file descriptor slots currently allocated
Groups supplementary group list
NStgid descendant namespace thread group ID hierarchy
VmPeak peak virtual memory size
VmSize total program size
VmLck locked memory size
+ VmPin pinned memory size
VmHWM peak resident set size ("high water mark")
VmRSS size of memory portions. It contains the three
following parts (VmRSS = RssAnon + RssFile + RssShmem)