Linux – Difference Between User Space and Kernel Space

driverskernellinux

Is Kernel space used when Kernel is executing on the behalf of the user program i.e. System Call? Or is it the address space for all the Kernel threads (for example scheduler)?

If it is the first one, than does it mean that normal user program cannot have more than 3GB of memory (if the division is 3GB + 1GB)? Also, in that case how can kernel use High Memory, because to what virtual memory address will the pages from high memory be mapped to, as 1GB of kernel space will be logically mapped?

Best Answer

Is Kernel space used when Kernel is executing on the behalf of the user program i.e. System Call? Or is it the address space for all the Kernel threads (for example scheduler)?

Yes and yes.

Before we go any further, we should state this about memory.

Memory get's divided into two distinct areas:

The user space, which is a set of locations where normal user processes run (i.e everything other than the kernel). The role of the kernel is to manage applications running in this space from messing with each other, and the machine.
The kernel space, which is the location where the code of the kernel is stored, and executes under.

Processes running under the user space have access only to a limited part of memory, whereas the kernel has access to all of the memory. Processes running in user space also don't have access to the kernel space. User space processes can only access a small part of the kernel via an interface exposed by the kernel - the system calls. If a process performs a system call, a software interrupt is sent to the kernel, which then dispatches the appropriate interrupt handler and continues its work after the handler has finished.

Kernel space code has the property to run in "kernel mode", which (in your typical desktop -x86- computer) is what you call code that executes under ring 0. Typically in x86 architecture, there are 4 rings of protection. Ring 0 (kernel mode), Ring 1 (may be used by virtual machine hypervisors or drivers), Ring 2 (may be used by drivers, I am not so sure about that though). Ring 3 is what typical applications run under. It is the least privileged ring, and applications running on it have access to a subset of the processor's instructions. Ring 0 (kernel space) is the most privileged ring, and has access to all of the machine's instructions. For example to this, a "plain" application (like a browser) can not use x86 assembly instructions lgdt to load the global descriptor table or hlt to halt a processor.

If it is the first one, than does it mean that normal user program cannot have more than 3GB of memory (if the division is 3GB + 1GB)? Also, in that case how can kernel use High Memory, because to what virtual memory address will the pages from high memory be mapped to, as 1GB of kernel space will be logically mapped?

For an answer to this, please refer to the excellent answer by wag here

Related Solutions

Linux – ZONE_NORMAL and it’s association with Kernel/User-pages

On a 32-bit architecture you have 0xffffffff (4'294'967'295 or 4 GB) linear addresses (not physical space) to refer to a physical address.
Even with only 512 MB of physical storage (the real RAM stick connected to the bus), the kernel will still use 4'294'967'295 (4 GB) linear addresses to calculate the physical ones.

The linux kernel divides these 4 GB (of addresses) into the user space (high memory) and the kernel space (low memory) by 3/1, so the kernel space has 1'073'741'823 (1 GB) of linear addresses to use.

These 1 GB of linear addresses, are only accessible by the kernel and are getting divided up even further.

ZONE_DMA: Contains page frames of memory below 16 MB. This is used for old ISA buses, they are able to address only the first 16 MB of RAM.

ZONE_NORMAL: Contains page frames of memory at and above 16 MB and below 896 MB, these are the addresses, which the kernel can map/access directly.

ZONE_HIGHMEM: Contains page frames of memory at and above 896 MB, page frames above this border are not generally mapped to the kernel space and therefore not directly accessible by the kernel. Page frames from the user space can be temporarily or permanently mapped here.

How much real, physical RAM space is occupied by the different zones depends on the form and number of processes you run.

If you enter free -ml in your console, you can see the usage including low- and high memory:

             total       used       free     shared    buffers     cached
Mem:          3022       2116        905          0        105       1342
Low:           839        196        642
High:         2182       1919        263
-/+ buffers/cache:        667       2354
Swap:         2859         93       2766

Linux – High memory (user space) and highmem (kernel space)

"High memory" and "low memory" do not apply to the virtual address space of processes, it's about physical memory instead.

In the process' virtual address space, the user space occupies the first 3GB, and the kernel space the fourth GB of this linear address space.

The first 896MB of the kernel space (not only kernel code, but its data also) is "directly" mapped to the first 896 MB of the physical memory. It is "direct" in the sense that there's always an offset of 0xc0000000 between any linear address of this 896MB part of the virtual kernel space and its corresponding address in physical memory (note however that the MMU is enabled and that page table entries are actually used for this).

The last 128MB part of the virtual kernel space is where are mapped some pieces of the physical "high memory" (> 896MB) : thus it can only map no more than 128MB of "high memory" at a time.

Reference: "Understanding the Linux Kernel", third edition - sections "8.1.3. Memory Zones" and "8.1.6. Kernel Mappings of High-Memory Page Frames".

Best Answer

Related Solutions

Linux – ZONE_NORMAL and it’s association with Kernel/User-pages

Linux – High memory (user space) and highmem (kernel space)

Related Question