What is kernel space in linux

Linux kernel space and user space

I’m confused about how exactly kernel and user space are structured and which portions of memory the occupy. My current (possibly wrong) understanding is this:

A process is created and this processes’ virtual memory is split up into a user-space and a kernel-space region, where as the user space region contains data, code, stack, heap etc. of the process and the kernel-space region contains things such as the page table for the process and kernel code. I’m not sure as to what the kernel code would be. driver code or similar stuff?

Also, is the system call table always mapped to the same region in the kernel-space of a process? (Is it even correct to say «kernel-space of a process»?

If I write my own driver/module and insert it, will that driver code then automatically be copied into the kernel-space of every new process that is created? If not. how exactly does this work?

Thanks in advance for any input, literature/links that can help clarify my questions are okay as well.

2 Answers 2

You’ve got the general idea mostly right, but make this adjustment: there’s only one «kernelspace» for the whole machine, and all processes share it.

When a process is active, it can either be running in «user mode» or «kernel mode».

In user mode, the instructions being executed by the CPU are in the userspace side of the memory map. The program is running its own code, or code from a userspace library. In user mode, a process has limited abilities. There is a flag in the CPU which tells it not to allow the use of privileged instructions, and kernel memory, although it exists in the process’s memory map, is inaccessible. (You wouldn’t want let any program just read and write the kernel’s memory — all security would be gone.)

When a process wants to do something other than move data around in its own (userspace) virtual memory, like open a file for example, it must make a syscall. Each CPU architecture has its own unique quirky method of making syscalls, but they all boil down to this: a magic instruction is executed, the CPU turns on the «privileged mode» flag, and jumps to a special address in kernelspace, the «syscall entry point».

Now the process is running in kernel mode. Instructions being executed are located in kernel memory, and they can read and write any memory they want to. The kernel examines the request that the process just made and decides what to do with it.

In the open example, the kernel receives 2 or 3 parameters corresponding to the arguments of int open(const char *filename, int flags[, int mode]) . The first argument provides an example of when kernelspace needs access to userspace. You said open(«foo», O_RDONLY) so the string «foo» is part of your program in userspace. The syscall mechanism only passed a pointer, not a string, so the kernel must read the string from user memory.

To find the requested file, the kernel may consult with filesystem drivers (to figure out where the file is) and block device drivers (to load the necessary blocks from disk) or network device drivers and protocols (to load the file from a remote source). All of those things are part of the kernel, i.e. in kernelspace, regardless of whether they are built-in or were loaded as modules.

If the request can’t be satisfied immediately, the kernel may put the process to sleep. That means the process will be taken off the CPU until a response is received from the disk or network. Another process may get a chance to run now. Later, when the response comes in, your process starts running again (still in kernel mode). Now that it’s found the file, the open syscall can finish up (check the permissions, create a file descriptor) and return to userspace.

Returning to userspace is a simple matter of putting the CPU back in non-privileged mode and restoring the registers to what they were before the user->kernel transition, with the instruction pointer pointing at the instruction after the magic syscall instruction.

Besides syscalls, there are other things that can cause a transition from user mode to kernel mode, including:

1. page faults — if your process accesses a virtual memory address that doesn’t have a physical address assigned to it, the CPU enters kernel mode and jumps to the page fault handler. The kernel then decides whether the virtual address is valid or not, and it either creates a physical page and resumes the process in userspace where it left off, or sends a SIGSEGV.
2. interrupts — some hardware (network, disk, serial port, etc.) notifies the CPU that it requires attention. The CPU enters kernel mode and jumps to a handler, the kernel responds to it and then resumes the userspace process that was running before the interrupt.

Loading a module is done with a syscall that asks the kernel to copy the module’s code and data into kernelspace and run its initialization code in kernel mode.

This is pretty long, so I’m stopping. I hope the walk-through focusing on user-kernel transitions has provided enough examples to solidify the idea.

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What is kernel space in Linux?

System memory in Linux can be divided into two distinct regions: kernel space and user space. Kernel space is where the kernel (i.e., the core of the operating system) executes (i.e., runs) and provides its services. A process is an executing instance of a program. …

What is difference between user space and kernel space?

Kernel space is strictly reserved for running a privileged operating system kernel, kernel extensions, and most device drivers. In contrast, user space is the memory area where application software and some drivers execute.

What does kernel mean in Linux?

The Linux® kernel is the main component of a Linux operating system (OS) and is the core interface between a computer’s hardware and its processes. It communicates between the 2, managing resources as efficiently as possible.

How do I get kernel space from user space?

One way I know is through a system call.

There are multiple system calls we can use, but at the end they are all system calls. Even in system calls, we send a data to kernel space, where it(driver or respective module) calls functions like copy_from_user() to copy data from user space to kernel space.

What does the kernel do?

The kernel connects the system hardware to the application software, and every operating system has a kernel. For example, the Linux kernel is used numerous operating systems including Linux, FreeBSD, Android, and others. … The kernel is responsible for: Process management for application execution.

What are different types of kernel?

Types of Kernel :

• Monolithic Kernel – It is one of types of kernel where all operating system services operate in kernel space. …
• Micro Kernel – It is kernel types which has minimalist approach. …
• Hybrid Kernel – It is the combination of both monolithic kernel and mircrokernel. …
• Exo Kernel – …
• Nano Kernel –

What is microkernel OS?

In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC).

What is difference between OS and kernel?

The basic difference between an operating system and kernel is that operating system is the system program that manages the resources of the system, and the kernel is the important part (program) in the operating system. … On the other hand, Opertaing system acts as an interface between user and computer.

Why is it called kernel?

The word kernel means “seed,” “core” in nontechnical language (etymologically: it’s the diminutive of corn). If you imagine it geometrically, the origin is the center, sort of, of a Euclidean space. It can be conceived of as the kernel of the space.

Is Linux kernel a process?

From the process management point of view, the Linux kernel is a preemptive multitasking operating system. As a multitasking OS, it allows multiple processes to share processors (CPUs) and other system resources.

Is user space faster than kernel space?

1 Answer. In general, code that runs in kernel space runs at the same speed as code in user space. … Where code can run faster in kernel space is when system calls are made. When user mode code calls a system function, the OS switches into supervisor mode, and this transition can be slow.

Do system programs run in kernel space?

The really simplified answer is that the kernel runs in kernel space, and normal programs run in user space. … The kernel is the core of the operating system. It normally has full access to all memory and machine hardware (and everything else on the machine).

Can kernel access user space memory?

Whilst a user-space program is not allowed to access kernel memory, it is possible for the kernel to access user memory. However, the kernel must never execute user-space memory and it must also never access user-space memory without explicit expectation to do so.

Is the kernel important in an operating system?

The operating system kernel represents the highest level of privilege in a modern general purpose computer. The kernel arbitrates access to protected hardware and controls how limited resources such as running time on the CPU and physical memory pages are used by processes on the system.

Does Windows have a kernel?

The Windows NT branch of windows has a Hybrid Kernel. It’s neither a monolithic kernel where all services run in kernel mode or a Micro kernel where everything runs in user space.

How does Linux kernel work?

The Linux kernel mainly acts as a resource manager acting as an abstract layer for the applications. The applications have a connection with the kernel which in turn interacts with the hardware and services the applications. Linux is a multitasking system allowing multiple processes to execute concurrently.

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What is «the kernel address space»?

From Understanding The Linux Kernel, here is some discussion about kernel thread vs user process i.e. regular process:

Besides user processes, Unix systems include a few privileged processes called kernel threads with the following characteristics:

• They run in Kernel Mode in the kernel address space.

• They do not interact with users, and thus do not require terminal devices.

• They are usually created during system startup and remain alive until the system is shut down.

In Linux, kernel threads differ from regular processes in the following ways:

• Kernel threads run only in Kernel Mode, while regular processes run alternatively in Kernel Mode and in User Mode.

• Because kernel threads run only in Kernel Mode, they use only linear addresses greater than PAGE_OFFSET. Regular processes, on the other hand, use all four gigabytes of linear addresses, in either User Mode or Kernel Mode.

I have heard about the virtual address space of a user process i.e. regular process, and a portion of the address space is mapped to the kernel code and data.

• I was wondering what «the kernel address space» in the above quote mean?
• Is it not the part of the virtual address space of a user process?
• Does it mean that the kernel have its own virtual address space, just like a user process has its own virtual address space?

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What is difference between User space and Kernel space?

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?

3 Answers 3

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)?

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

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What are high memory and low memory on Linux?

I’m interested in the difference between Highmem and Lowmem:

1. Why is there such a differentiation?
2. What do we gain by doing so?
3. What features does each have?

7 Answers 7

On a 32-bit architecture, the address space range for addressing RAM is:

or 4’294’967’295 (4 GB).

The linux kernel splits that up 3/1 (could also be 2/2, or 1/3 1 ) into user space (high memory) and kernel space (low memory) respectively.

The user space range:

Every newly spawned user process gets an address (range) inside this area. User processes are generally untrusted and therefore are forbidden to access the kernel space. Further, they are considered non-urgent, as a general rule, the kernel tries to defer the allocation of memory to those processes.

The kernel space range:

A kernel processes gets its address (range) here. The kernel can directly access this 1 GB of addresses (well, not the full 1 GB, there are 128 MB reserved for high memory access).

Processes spawned in kernel space are trusted, urgent and assumed error-free, the memory request gets processed instantaneously.

Every kernel process can also access the user space range if it wishes to. And to achieve this, the kernel maps an address from the user space (the high memory) to its kernel space (the low memory), the 128 MB mentioned above are especially reserved for this.

1 Whether the split is 3/1, 2/2, or 1/3 is controlled by the CONFIG_VMSPLIT_. option; you can probably check under /boot/config* to see which option was selected for your kernel.

The first reference to turn to is Linux Device Drivers (available both online and in book form), particularly chapter 15 which has a section on the topic.

In an ideal world, every system component would be able to map all the memory it ever needs to access. And this is the case for processes on Linux and most operating systems: a 32-bit process can only access a little less than 2^32 bytes of virtual memory (in fact about 3GB on a typical Linux 32-bit architecture). It gets difficult for the kernel, which needs to be able to map the full memory of the process whose system call it’s executing, plus the whole physical memory, plus any other memory-mapped hardware device.

So when a 32-bit kernel needs to map more than 4GB of memory, it must be compiled with high memory support. High memory is memory which is not permanently mapped in the kernel’s address space. (Low memory is the opposite: it is always mapped, so you can access it in the kernel simply by dereferencing a pointer.)

When you access high memory from kernel code, you need to call kmap first, to obtain a pointer from a page data structure ( struct page ). Calling kmap works whether the page is in high or low memory. There is also kmap_atomic which has added constraints but is more efficient on multiprocessor machines because it uses finer-grained locking. The pointer obtained through kmap is a resource: it uses up address space. Once you’ve finished with it, you must call kunmap (or kunmap_atomic ) to free that resource; then the pointer is no longer valid, and the contents of the page can’t be accessed until you call kmap again.

This is relevant to the Linux kernel; I’m not sure how any Unix kernel handles this.

The High Memory is the segment of memory that user-space programs can address. It cannot touch Low Memory.

Low Memory is the segment of memory that the Linux kernel can address directly. If the kernel must access High Memory, it has to map it into its own address space first.

There was a patch introduced recently that lets you control where the segment is. The tradeoff is that you can take addressable memory away from user space so that the kernel can have more memory that it does not have to map before using.

HIGHMEM is a range of kernel’s memory space, but it is NOT memory you access but it’s a place where you put what you want to access.

A typical 32bit Linux virtual memory map is like:

0x00000000-0xbfffffff: user process (3GB)

0xc0000000-0xffffffff: kernel space (1GB)

(CPU-specific vector and whatsoever are ignored here).

Linux splits the 1GB kernel space into 2 pieces, LOWMEM and HIGHMEM. The split varies from installation to installation.

If an installation chooses, say, 512MB-512MB for LOW and HIGH mems, the 512MB LOWMEM (0xc0000000-0xdfffffff) is statically mapped at the kernel boot time; usually the first so many bytes of the physical memory is used for this so that virtual and physical addresses in this range have a constant offset of, say, 0xc0000000.

On the other hand, the latter 512MB (HIGHMEM) has no static mapping (although you could leave pages semi-permanently mapped there, but you must do so explicitly in your driver code). Instead, pages are temporarily mapped and unmapped here so that virtual and physical addresses in this range have no consistent mapping. Typical uses of HIGHMEM include single-time data buffers.

For the people looking for an explanation in the context of Linux kernel memory space, beware that there are two conflicting definitions of the high/low memory split (unfortunately there is no standard, one has to interpret that in context):

«High memory» defined as the totality of kernel space in VIRTUAL memory. This is a region that only the kernel can access and comprises all virtual addresses greater or equal than PAGE_OFFSET . «Low memory» refers therefore to the region of the remaining addresses, which correspond to the user-space memory accessible from each user process.

For example: on 32-bit x86 with a default PAGE_OFFSET , this means that high memory is any address ADDR with ADDR ≥ 0xC0000000 = PAGE_OFFSET (i.e. higher 1 GB).

This is the reason why in Linux 32-bit processes are typically limited to 3 GB. Note that PAGE_OFFSET cannot be configured directly, it depends on the configurable VMSPLIT_x options (source).

To summarize: in 32-bit archs, virtual memory is by default split into lower 3 GB (user space) and higher 1 GB (kernel space).

For 64 bit, PAGE_OFFSET is not configurable and depends on architectural details that are sometimes detected at runtime during kernel load.

On x86_64, PAGE_OFFSET is 0xffff888000000000 for 4-level paging (typical) and 0xff11000000000000 for 5-level paging (source). For ARM64 this is usually 0x8000000000000000 . Note though, if KASLR is enabled, this value is intentionally unpredictable.

«High memory» defined as the portion of PHYSICAL memory that cannot be mapped contiguously with the rest of the kernel virtual memory. A portion of the kernel virtual address space can be mapped as a single contiguous chunk into the so-called physical «low memory». To fully understand what this means, a deeper knowledge of the Linux virtual memory space is required. I would recommend going through these slides.

From the slides:

This kind of «high/low memory» split is only applicable to 32-bit architectures where the installed physical RAM size is relatively high (more than

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