musl - confusion about brk and mmap in expand

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Message-ID: <CAAKbDrfwRxP_yQLapYdhQR1ubDhnZBo0ebCQk7e02L8A0HHDHw@mail.gmail.com>
Date: Thu, 10 Sep 2015 14:58:33 -0400
From: Yuxin Ren <ryx@...ail.gwu.edu>
To: musl@...ts.openwall.com
Subject: confusion about brk and mmap in expand_heap in malloc implementation

Hi,

I found in the __expand_heap function used by malloc, if brk fails, it will
use mmap to get a memory area.
I think in this case, this area should be returned back to system via
munmap.
But in my test, when I free such area, I found it is still passed to
madvise, not munmap.

How do we track if an area is expanded by brk or mmap?
And when we free an area, how do we decide to use munmap or madvise?

Thanks a lot.
Yuxin

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