What is pg backend using memory for?

Hi,
So, I checked a backend on Linux, and found such thing:
2ba63c797000-2ba63fa68000 rw-p 2ba63c797000 00:00 0
Size: 52036 kB
Rss: 51336 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 51336 kB
Swap: 0 kB
Pss: 51336 kB

(this is part of /proc/<pid>/smaps).

This is not shared memory, so it's local. It's not related to any files (in such case first line would have path to file).

What's more - this backend, during getting smaps copy was idle, and it's not stats manager, or anything like this.

How can this be diagnosed, to find out why there is so much private
memory?

In case it matters: it's pg 9.1.6 on linux 2.6.18-164.2.1.el5

Best regards,

depesz

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