How kernel threaduse memory descriptor(mm_struct) of last ran process in Linux?

Question

Some of the points mentioned in the Linux kernel Development (by Robert Love) book about mm_struct and kernel thread are :

"Kernel threads do not have a process address space and therefore do not have an associated memory descriptor. Thus, the mm field of a kernel thread's process descriptor is NULL. "

"Because kernel threads do not have any pages in user-space, they do not really deserve their own memory descriptor and page tables (page tables are discussed later in the chapter). Despite this, kernel threads need some of the data, such as the page tables, even to access kernel memory."

"Kernel threads do not have an address space and mm is NULL. Therefore, when a kernel thread is scheduled, the kernel notices that mm is NULL and keeps the previous process's address space loaded. The kernel then updates the active_mm field of the kernel thread's process descriptor to refer to the previous process's memory descriptor. The kernel thread can then use the previous process's page tables as needed."

Now my queries are: 1. First it is mentioned that the kernel threads dont have any page in user space and hence they dont deserve memory desriptor and page tables and in the next line it says it needs some data such as page tables to access kernel memory. What page table it is referring here?? Every process has its own page table for mapping the virtual to physical address, why kernel thread requires that?

How page table use by kernel thread?

Answer 1

Every thread whether its a user-space or kernel space process requires a page-table. The kernel address space (Virtual Memory address space) is directly mapped to the physical address space where as the user-space address space isn't directly mapped. Moreover the user-space application address space mappings keeps changing as the new processes are created, terminated, swapped whereas the kernel space mappings remain constant. To learn more you can visit the following link :-

Process address Space

Or post the queries here.

Answer 2

There are some usespace applications and kernel threads in your system. Every virtual address space consists of kernel and user part. Kernel is the same for all processes, user part is different.

Every process has its own page table for mapping the virtual to physical address, why kernel thread requires that?

Kernel threads need page tables to make translation from virtual address to physical while accessing memory.

How page table use by kernel thread?

Imagine a simple case, memory write such as a[i] = 5; in kernel space. This one typically goes through MMU, which use page tables to get physical address according to virtual address (in this case &a[i]). So there is no something special about kernel threads, the difference is that on context switch they don't change pgd (page global directory), they use pgd of last process, because all processes have the same kernel part and you can pick just last one (see actime_mm) and it will be ok.