How do you communicate between user and kernel space?

There are couple of well known methods to communicate from user space to kernel space.

1. Virtual file system like /proc , /sys , /configfs , /debugfs Standard.
2. system call like read() , write() , open() , close() , fork()
3. ioctl for char drivers.
4. netlink socket – mostly used by network subsystem.

Does mmap allocate memory?

mmap() is a system call that can be used by a user process to ask the operating system kernel to map either files or devices into the memory (i.e., address space) of that process. The mmap() system call can also be used to allocate memory (an anonymous mapping).

Does mmap allocate physical memory?

In theory, neither mmap nor malloc allocate actual RAM, but in practice they do. mmap will allocate RAM to store a virtual memory area data structure (VMA).

Who established communication between user and kernel in Linux?

Then, Alexey Kuznetsov extended it during Linux kernel 2.1 development to provide a flexible and extensible messaging interface to the new advanced routing infrastructure. Since then, Netlink sockets have become one of the main interfaces that kernel subsystems provide to user-space applications in Linux.

What is Copy_to_user in Linux?

The copy_to_user function copies a block of data from the kernel into user space. This function accepts a pointer to a user space buffer, a pointer to a kernel buffer, and a length defined in bytes. The function returns zero on success or non-zero to indicate the number of bytes that weren’t transferred.

What is kernel address space?

The book uses the term “kernel address space” to refer to the partition of the virtual address space that is allocated for the kernel. Recently, Linux and other OSes have implemented page-table isolation (PTI) to mitigate the Meltdown security vulnerability.

How do I allocate more memory with mmap?

You can use mmap to allocate an area of private memory by setting MAP_ANONYMOUS in the flags parameter and setting the file descriptor fd to -1 . This is similar to allocating memory from the heap using malloc , except that the memory is page-aligned and in multiples of pages.

How to implement mmap with non-swappable kernel memory?

One way to implement mmap would be to use remap_pfn_range but LDD3 says this does not work for conventional memory. It appears we can work around this by marking the page (s) reserved using SetPageReserved so that it gets locked in memory. But isn’t all kernel memory already non-swappable i.e. already reserved?

How is user memory managed in Linux?

This article begins with a discussion of memory management within Linux, and then explores the methods for manipulation of user address space from the kernel. In Linux, user memory and kernel memory are independent and implemented in separate address spaces.

What is memory mapping in Linux kernel?

Memory Mappings. The Linux kernel works with different memory mappings. One mapping, called kernel virtual mapping provides a direct 1 to 1 mapping of physical addresses to virtual addresses. Conversion between physical and virtual addresses can be made with phys_to_virt() and virt_to_phys() for the translation virtual to physical.

What is the relationship between kernel and user memory in Linux?

In Linux, user memory and kernel memory are independent and implemented in separate address spaces. The address spaces are virtualized, meaning that the addresses are abstracted from physical memory (through a process detailed shortly).