The Linux memory address space layout

In this lesson, you will learn how the Linux kernel organizes the virtual address space to manage the divide between user-mode applications and the kernel.

The Virtual Memory Map

When a process runs on Linux, it operates within a "virtual" memory address space. This creates an illusion that the process owns the entire system's memory, isolating it from other processes and protecting the kernel. On a typical 64-bit architecture, this space is split into two primary segments: the User Space (the lower half) and the Kernel Space (the upper half).

Visualizing the Layout

From the perspective of a user-space process, the memory layout generally looks like this (from lowest to highest address):

+-----------------------+ 0x0000000000000000
| Text (Code) Segment   |
+-----------------------+
| Data/BSS (Variables)  |
+-----------------------+
| Heap (Dynamic memory) | Grows Upward
+-----------------------+
| Memory Mappings (MMAP)| Shared libraries, files
+-----------------------+
| Stack (Local vars)    | Grows Downward
+-----------------------+
| ... Gap (No access) ...|
+-----------------------+
| Kernel Space          | 0xFFFFFFFFFFFFFFFF
+-----------------------+

The Sections Explained

• Text/Data: These contain your compiled binary code and global variables. They occupy the fixed, lowest part of the memory.
• Heap: Used for dynamic allocation (e.g., malloc in C). It grows toward higher addresses as the application requests more memory.
• Memory Mappings: This region hosts shared libraries (like libc) and memory-mapped files. It provides a flexible area for the kernel to map file contents directly into process memory.
• Stack: Used for function calls, local variables, and return addresses. Crucially, the stack grows downward toward the heap. If they meet, you experience a "stack overflow."
• Kernel Space: This area is invisible to the user application. When an application performs a system call, the CPU switches privilege levels, allowing it to execute code within this reserved upper memory region. The kernel map is consistent across all processes, ensuring that the kernel is always available to handle interrupts and hardware requests.

Key Takeaway: The Linux memory layout segregates processes into a private, lower virtual address space while reserving a fixed, global upper address space for the kernel to maintain system-wide control and security.