user space vs kernel space address space

Miles MH Chen orca.chen at
Sun Dec 29 08:34:33 EST 2013

Hi Pritam,

1) Yes, all 512RAM will be direct mapped to kernel address space IF the
kernel have a 896MB direct mapping area.
Actually you can change the range of kernel direct mapping by the
vmalloc=<size> in boot command line.
In 32-bit and 3:1 split configuration, kernel direct mapping area + vmalloc
area is roughly 1G.

2) User space and kernel space can have different virtual addresses mapping
to the same physical frame at the same time.

3) Direct mapped or fixed map or permanent map are describing kernel
VIRTUAL address space, you can see the virtual memory
layout by 'dmesg'. When user space needs memory, the kernel allocates a
free memory frame, and remaps the frame to user space.
The frame does not have to have a valid kernel virtual address.


On Sun, Dec 29, 2013 at 5:42 PM, Pritam Bankar
<pritambankar1988 at>wrote:

> Lets consider 32 bit Linux system with 512 physical RAM. Suppose I have
> standard 3:1 address space split. Now what I understand is
> (In general)
> 1. In the fourth gigabyte I have kernel space
> 2. Out of 1GB for kernel address space only 896MB is used as direct
> mapping and other 128 MB is used for Noncontiguous Memory Area Management,
> Fixed Mapping and Permanent Mapping.
> Following are my doubts :
> 1. Since my system has only 512MB RAM, will there be only direct mappings
> since 896 is enough to hold 512 RAM?
> 2. When user space program do malloc, we get some virtual address from
> userspace region (from first 3GB) of process. So will it be like, when I
> access some memory from that region, there is some physical frame
> associated with it AND same physical frame will also be mapped in kernel
> space ?
> So what I want  to know, for every physical frame is there a mapping in
> userspace as well as kernel space ? (given that some address in user space
> of process map to same physical frame)
> 3. If we consider example of Linux system with 4GB, now if I do malloc
> from user space from which memory region kernel will give memory ? Direct
> mapped region or fixed map or permanent map ?
> Thanks and regards,
> Pritam Bankar
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at
-------------- next part --------------
An HTML attachment was scrubbed...

More information about the Kernelnewbies mailing list