How Linux Kernel Turns malloc Calls into Physical Memory: A Simple Guide

Memory Allocation Overview

The article starts with the familiar malloc interface that user programs call to obtain memory. In Linux, malloc returns a virtual address; the corresponding physical memory is not allocated at this point.

Virtual Memory vs Physical Memory

Virtual memory is a logical concept from the process perspective and does not exist as a physical entity.

Physical memory resides on the actual RAM modules.

Each virtual address has a potential mapping to a physical address.

When virtual memory is allocated, the physical memory may remain unassigned until it is accessed.

Page Tables

Mapping between virtual and physical memory is maintained by page tables, which can be thought of as a hash‑like table stored in memory that records the correspondence of virtual addresses to physical frames.

Who Uses Page Tables?

The page table is not accessed directly by applications. Instead, the CPU’s Memory Management Unit (MMU) reads the page table to translate virtual addresses to physical ones. The operating system kernel creates and updates the page tables, while the MMU handles the actual translation.

Multiple Page Tables per Process

Each process has its own independent virtual address space, so each process requires its own page table. Consequently, the number of page tables in the system equals the number of active processes.

Page Table Creation and Maintenance

Page tables are created when a new process is spawned. The kernel is responsible for building and maintaining these tables throughout the process’s lifetime.

Page Fault Handling

If the MMU looks up a virtual address and finds no corresponding page‑table entry, it triggers a page‑fault exception. The hardware raises this synchronous exception, and the kernel’s page‑fault handler allocates a physical page, updates the page table, and then resumes the original instruction.

Step‑by‑Step Memory Allocation Process

User‑space program calls malloc and receives a virtual address.

The program later accesses that address (e.g., via memset).

The MMU attempts to translate the virtual address to a physical one.

The MMU reads the page table and discovers the entry is missing.

A page‑fault exception is automatically generated by the hardware.

The CPU jumps to the kernel’s registered page‑fault handler.

The handler allocates a physical page and inserts a new entry into the process’s page table.

Control returns to user space, and the original memory operation proceeds.

Further Topics

Kernel memory allocators such as slab and vmalloc The buddy system for managing physical pages

Process address‑space management