In Japanese server operating systems, processes are the basic unit of program execution. The kernel stack and process structure memory are crucial core concepts, fundamental to understanding how Japanese server operating systems manage, schedule, and isolate multiple programs.

The process structure memory is the operating system's "process management center." In the Linux kernel, this structure is called `task_struct`. It's a tightly managed memory area containing all the information the operating system needs to manage and control a process. It mainly includes the following categories:

Identification information: such as process ID (PID) and parent process ID (PPID), used to uniquely identify and establish family relationships between processes.

Status and scheduling information: records whether the process is currently running, waiting for resources, or has stopped. It also includes priority, scheduling policy, and cumulative running time, allowing the CPU scheduler to decide which process should be executed on the CPU next.

Memory management information: This is the key part. It stores pointers to the process's memory mappings, i.e., page tables. Page tables define the actual locations of the process's code, data, stack, and other parts in physical memory, serving as the legal basis for a process to have its own independent "memory space."

File and Resource Information: Records which files a process has opened, which semaphores, locks, and other resources it uses, ensuring proper release upon process termination and preventing resource leaks.

Processor Context: When a process is switched out of the CPU due to time slice expiration or waiting for an event, the values ​​of all its registers (including the program counter, PC) are saved here. When it is scheduled for execution again, its context is restored from here, achieving seamless continuation.

Simply put, `task_struct` is the operating system's "complete portrait" and "control panel" of a process. The kernel controls the entire process by traversing the linked list of all processes' `task_struct`.

The kernel stack of a process is its "private workbench" for executing kernel code. Each process has two stack memory areas: the user stack and the kernel stack.

User Stack: Located in the process's user address space, used for function calls, storing local variables, etc., and used by the program itself.

Kernel Stack: Independent and crucial, it resides in the kernel address space and is a private workspace specifically for the process to use when executing kernel code.

Why is a separate "kernel stack" needed? This stems from the CPU's operating mode. Modern CPUs typically have two privilege levels: user mode and kernel mode. User programs run in restricted user mode and cannot directly access hardware. When a program needs to perform system calls (such as reading/writing files, allocating memory), or respond to hardware interrupts, the CPU must switch to the higher-privilege kernel mode to execute code provided by the kernel.

The core function of the kernel stack is to provide temporary memory space for kernel code execution during such switches. Its workflow is as follows:

Context Switching: When switching from user mode to kernel mode, the CPU automatically pushes some user-mode registers (such as the instruction pointer and stack pointer) onto the current process's kernel stack and switches to the kernel's code segment for execution.

Kernel Function Calls: During kernel code execution, its internal function calls and local variables also require stack space for storage. This information is pushed onto and used in this dedicated, private kernel stack, rather than a shared area.

Security and Isolation: Each process has its own kernel stack. This ensures that when the kernel processes a system call from process A, its temporary data is not mixed with that of process B. This is the cornerstone of system security and stability. If all processes shared a single kernel stack, the data would be completely corrupted.

A vivid scenario: Suppose process A writes data to a file using the `write()` system call. The `write()` library prepares the call parameters and then triggers a special instruction (such as `int 0x80` or `syscall`) to cause the CPU to jump to the kernel. At this point, the hardware automatically saves the user-mode context to process A's kernel stack and then jumps to the kernel's "system call handler" for execution. During execution, this handler uses process A's kernel stack for its function call chain and temporary variables. After processing, it restores the user-mode context from the kernel stack and returns to process A to continue execution.

The kernel stack and process structure are a collaborative dual-core architecture. We can connect these two parts:

Kernel perspective: The kernel manages and identifies a process globally through `task_struct` (process structure).

From an execution perspective: When a process needs kernel services, the execution context and data of the kernel code are stored in the process's dedicated kernel stack.

In fact, within Linux's `task_struct` structure, there is a pointer that directly points to the bottom (or top) of the process's kernel stack. This means that given any `task_struct`, the kernel can immediately find the corresponding kernel stack; conversely, when the CPU is handling an interrupt or system call, it can also use the current kernel stack pointer to find which process's `task_struct` is currently being served.

In short, the process structure's memory (`task_struct`) is the process's static management file. It exists from birth to death, defining who the process is, what resources it has, and what state it is in. The process's kernel stack is the process's private workbench when dynamically executing kernel code. It is only actively used when the process enters kernel mode to save temporary data for context switching and kernel function calls.

Understanding these two is crucial for understanding operating systems. These are the underlying foundations for core mechanisms such as process isolation (each process feels it has exclusive access to the CPU and memory), CPU scheduling (process switching via saving/restoring context), and system calls (safely transitioning from user mode to kernel mode).

For developers, understanding the kernel stack helps in debugging complex problems (such as analyzing kernel Oops logs); understanding process structure provides a deeper understanding of the meaning of information output by system performance tools (such as `top` and `ps`), enabling more effective system optimization.