SQL> select statistic#, name from v$statname where name like '%pga%' or name like '%uga%';

STATISTIC# NAME
---------- ----------------------------------------------------------------
        15 session uga memory
        16 session uga memory max
        20 session pga memory
        21 session pga memory max

SQL> select * from v$sesstat where statistic# in (15,16,20,21) and sid = 15;

       SID STATISTIC#      VALUE
---------- ---------- ----------
        15         15      77004
        15         16      77004
        15         20     243208
        15         21     243208

declare
  type array is table of varchar2(10) index by binary_integer;
  data array;
begin
  data(1) := 'A';
end;
/

SQL> select * from v$sesstat where statistic# in (15,16,20,21) and sid = 15;

       SID STATISTIC#      VALUE
---------- ---------- ----------
        15         15      77004
        15         16      77004
        15         20     243208
        15         21     308744

declare
  type array is table of varchar2(32767) index by binary_integer;
  data array;
begin
  for i in 1..1000000 loop
    data(i) := 'A';
  end loop;
end;
/

SQL> select * from v$sesstat where statistic# in (15,16,20,21) and sid = 15;

       SID STATISTIC#      VALUE
---------- ---------- ----------
        15         15      77004
        15         16      77004
        15         20    1656612
        15         21    1722148

declare
  type array is table of varchar2(32767) index by binary_integer;
  data array;
begin
  for i in 1..1000000 loop
    data(i) := '12345678901234567890123456789012345678901234567890';
  end loop;
end;
/

SQL> select * from v$sesstat where statistic# in (15,16,20,21) and sid = 15;

       SID STATISTIC#      VALUE
---------- ---------- ----------
        15         15      77004
        15         16      77004
        15         20   91534200
        15         21   91599736

SQL> exec dbms_session.free_unused_user_memory

PL/SQL procedure successfully completed.

SQL> select * from v$sesstat where statistic# in (15,16,20,21) and sid = 15;

       SID STATISTIC#      VALUE
---------- ---------- ----------
        15         15      77004
        15         16      77004
        15         20     251720
        15         21   91861880

SQL> select PGA_USED_MEM,PGA_ALLOC_MEM,PGA_FREEABLE_MEM,PGA_MAX_MEM from v$process where addr = '53E0B7B8';

PGA_USED_MEM PGA_ALLOC_MEM PGA_FREEABLE_MEM PGA_MAX_MEM
------------ ------------- ---------------- -----------
      209901        528181                0    91876197

But at OS level, cat /proc/16238/maps still shows

...
50000000-57000000 rw-s 00000000 00:04 4227080    /SYSV1be4c7a8 (deleted)
57000000-57001000 r--s 07000000 00:04 4227080    /SYSV1be4c7a8 (deleted)
57001000-570a2000 rw-s 07001000 00:04 4227080    /SYSV1be4c7a8 (deleted)
570a2000-570a3000 r--s 070a2000 00:04 4227080    /SYSV1be4c7a8 (deleted)
570a3000-57400000 rw-s 070a3000 00:04 4227080    /SYSV1be4c7a8 (deleted)
bffd7000-bffee000 rwxp fffea000 00:00 0