Why memory.usage_in_bytes is always increasing after every mmap/dirty/unmap sequence

[bill4carson <bill4carson@gmail.com>]

Hi, all

I'm playing with memory cgroup, I'm a bit confused why
memory.usage in bytes is steadily increasing at 4K page pace
after every mmap/dirty/unmap sequence.

On linux-3.6.34.10/linux-3.3.0-rc5
A simple test case does following:

a) mmap 128k memory in private anonymous way
b) dirty all 128k to demand physical page
c) print memory.usage_in_bytes  <-- increased at 4K after every loop
d) unmap previous 128 memory
e) goto a) to repeat


And when the test case exit, memory.usage_in_bytes is not *ZERO*, but
the previous increased value.

I'm puzzled about what I saw, can anyone please give me some tips
to understand this?


Thanks in advance.

-- 
Love each day!

--bill

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