I have 2 questions.

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1. Why physical address is greater than the installed memory?
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-- Installed memory is 8GB

RHEL7# grep MemTotal /proc/meminfo
MemTotal: 7840900 kB

RHEL7# dmidecode | grep -A 5 'Memory Device' | grep Size
Size: 4096 MB
Size: No Module Installed
Size: 4096 MB
Size: No Module Installed


-- Physical address is greater than 8GB

RHEL7# wc -c /proc/kpageflags
19136504 /proc/kpageflags

19136504 / 8Bytes x 4KB = 9.125GB


-- My program which is using pagemap shows following.

Phy_addr : 247f1e000 Virt_addr : 7f177ce2e000 systemd-journal(PID:514)


-- crash shows same physical address above 8GB!!.

crash> vtop -c 514 7f177ce2e000
VIRTUAL PHYSICAL
7f177ce2e000 247f1e000

247f1e000 = 9G 127M 120K


Why physical address is greater than installed memory?
Why is it possible?



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2. What is convenient use of pagemap for troubleshooting?
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For example, we can use the USS(Unique Set Size) to invest process memory leak.

Calculating USS by using pagemap
/proc/PID/pagemap has physical address and kpagecount is 1.
USS = Total of above condition pages x 4KB.

But, we can do the same thing by using smaps.
cat /proc/PID/smaps | awk '/^Private/{sum += $2}END{print sum}'



What is convenient use of pagemap for troubleshooting? (It's difficult by smaps)

1) Computers have advanced a little since 8-bit microprocessor days. Physical memory is made up of multiple segments. You can see them being enumerated by doing:

2) I've been doing kernel and driver development for quite a while and have never used pagemap, but I guess you can always learn new things. The only time I ever cared about a physical address was debugging a DMA problem on fibre channel cards (32-bit wraparound problem). You don't like virtual addresses for some reason?

1 members found this post helpful.

Thanks for reply.

1) e820 became very study for me.
Intel Memory Reclaiming function maps Hardware address at Memory Hole(around 3GB - 4GB), and remaps this RAM above 4GB.
My question was solved.

2) I'm sorry for less words.
I am excited that it is possible to know the physical address.
But my boss said what can this use for? Because my job is a trouble shooting for SIer departments within a company.
I am searching convenient use for memory troubleshooting.