In Linux each process has its virtual address space (e.g. 4 GB in case of 32 bit system, wherein 3GB is reserved for process and 1 GB for kernel). This virtual addressing mechanism helps isolating the address space of each process. This is understandable in case of process since there are many processes. But since we have 1 kernel only so why do we need virtual addressing for kernel?
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2 Answers
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The reason the kernel is "virtual" is not to deal with paging as such, it is becuase the processor can only run in one mode at a time. So once you turn on paged memory mapping (Bit 31 in CR0 on x86), the processor is expecting ALL memory accesses to go through the page-mapping mechanism. So, since we do want to access the kernel even after we have enabled paging (virtual memory), it needs to exist somewhere in the virtual space.
The "reserving" of memory is more about "easy way to determine if an address is kernel or user-space" than anything else. It would be perfectly possible to put a little bit of kernel at address 12345-34121, another bit of kernel at 101900-102400 and some other bit of kernel at 40000000-40001000. But it would make life difficult for every aspect of the kernel and userspace - there would be gaps/holes to deal with [there already are such holes/gapes, but having more wouldn't exactly help things]. By setting a fixed limit for "userspace is from here to here, kernel is from end of userspace to X", it makes life much easier in that respect. We can just say kernel = 0; if (address > max_userspace) kernel=1; in some code.
Of course, the kerneln only takes up as much PHYSICAL memory as it will actually use - so the common thinking that "it's a waste to take up a whole gigabyte for the kernel" is wrong - the kernel itself is a few (a dozen or so for a very "big" kernel) megabytes. The modules loaded can easily add up to several more megabytes, and graphics drivers from ATI and nVidia easily another few megabytes just for the kernel moduel for that itself. The kernel also uses some bits of memory to store "kernel data", such as tasks, queues, semaphores, files and other "stuff" the kernel has to deal with. A few megabytes is used for this as well.
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Virtual Memory Management is that feature of Linux which enables Multi-tasking in system without any limitation on no. of task or amount of memory used by each task. The Linux Memory Manager Subsystem (along with MMU hardware) facilitates VMM support, where memory or mem-mapped device are accessed through virtual addresses. Within Linux everything, both kernel and user components, works with virtual address except when dealing with real hardware. That's when the Memory Manager takes its place, does virtual-to-physical address translation and points to physical mem/dev location.
A process is an abstract entity, defined by kernel to which system resources are allocated in order to execute a program. In Linux Process Management the kernel is an integrated part of a process memory map. A process has two main regions, like two faces of one coin:
User Space view - contains user program sections (Code, Data, Stack, Heap, etc...) used by process
Kernel Space view - contains kernel data structures that maintain information (PID. States, FD, Resource Usage, etc...) about the process
Every process in Linux system has a unique and separate User Space Region. This feature of Linux VMM isolates each process program sections from one and other. But all processes in the system shares the common Kernel Space Region. When a process needs service from the kernel it must execute the kernel code in this region, or in other words kernel is performing on behalf of user process request.
