Anonymous mapped regions increases unlimitely on spawn

[unDEFER]

Hello!
I have the program which uses BDB and while testing often makes 
spawn. And after 12 hours of testing bdb said:
mmap: Cannot allocate memory

But the problem that I've found that it is not BDB created too 
many maps. Watching for /proc/[PID]/maps shows that number of 
anonymous mapped regions increases on 2 every spawn process, and 
never decreases even after finishing the spawned thread.
According to logs my program made 32543 spawns for test time. And 
my /proc/sys/vm/max_map_count = 65530. So only 444 maps was 
allocated by other reasons and 65086 by spawn.

So what to do? How to make spawn decrease count of anonymous 
mapped regions?