Linux MM : virtual memory address space

[Jeff Haran]

From: kernelnewbies-bounces at kernelnewbies.org [mailto:kernelnewbies-bounces at kernelnewbies.org] On Behalf Of pramod gurav
Sent: Monday, March 03, 2014 11:33 PM
To: priyaranjan
Cc: kernelnewbies
Subject: Re: Linux MM : virtual memory address space



On Mon, Mar 3, 2014 at 11:52 PM, priyaranjan <priyaranjan45678 at gmail.com<mailto:priyaranjan45678 at gmail.com>> wrote:
I was going through http://linux-mm.org/HighMemory

"Currently the 32 bit x86 architecture is the most popular type of computer. In this architecture, traditionally the Linux kernel has split the 4GB of virtual memory address space into 3GB for user programs and 1GB for the kernel"

What about 64-bit system? Where does the code lie in linux kernel for the check?
Is there any latest and updated memory management documentation for Linux kernel?

Regards,
Priyaranjan


Priyaranjan,

As below link suggests:
http://users.nccs.gov/~fwang2/linux/lk_addressing.txt

Also read this blog written in chinese:

http://adam8157.info/blog/2012/07/linux-x86-64-vm/

on 64 bit arch the virtual address space is huge (2 to thr power of 64). So the overhead of translating the virtual addresses will be high. TO avoid this only lower 48 bits are used to form virtual addresses.

I believe this statement about only the lower 48 bits being used it not correct. That would imply that the upper 16 bits of all virtual addresses on x86_64 would be the same, which is clearly not the case since the upper 16 bits of user space vas are all 0s yet the upper 16 bits of kernel space vas are all 1s.

Jeff Haran


This virtual space will still be very large (256 TB) and hence the user/kernel split is 1:1 (128TB:128TB). And as suggested in link it is very unlikely that you will have such a huge RAM(more than 128TB) installed on any machine there is no concept of HIGHMEM.

Hope this clears the air a bit.

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Thanks and Regards
Pramod
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