What You Will Learn

Prerequisites

What Is the Linux Kernel Page Allocator?

The Linux kernel manages physical RAM in fixed-size units called page frames (commonly 4 KB on x86 and ARM). Whenever any part of the kernel — the core, a loaded module, or a device driver — needs physically contiguous memory, it cannot simply call something like a user-space malloc(). Instead, it goes through the kernel’s own memory engine, and the lowest-level engine in that chain is the page allocator, implemented using an algorithm known as the buddy system.

The buddy system’s job is simple to state but clever in implementation: keep track of which page frames are free, and hand out physically contiguous blocks of pages efficiently, while minimizing wasted space (fragmentation) as much as possible.

How the Buddy System Freelist Works

The buddy allocator keeps an array of freelists, where each index in the array is called an order. Order N holds free blocks made up of 2^N contiguous page frames. This means the size of memory available on each list doubles as you move to the next order, and halves as you move to the previous one.

On most common architectures such as x86-64 and ARM64, the maximum order is 10 (an internal constant in current kernels sets the number of usable orders so that the largest single contiguous block obtainable is 2¹⁰ = 1024 pages, i.e. 4 MB with a 4 KB page size). Larger systems with different page-size capabilities can support a different range, but the underlying power-of-2 freelist principle stays the same across kernel versions.

Each entry in a freelist is not the raw memory itself — it is a kernel metadata structure (struct page) that describes and points to a real block of physical RAM. Two free blocks of the same order that sit next to each other in physical memory are called buddies. When both buddies become free at the same time, the allocator merges (coalesces) them into a single free block of the next higher order. This coalescing behavior is what keeps fragmentation low over time.

Inspecting the Buddy Allocator on a Running System

Modern Linux exposes a live summary of buddy allocator state through the proc filesystem. You can check it yourself on any Linux machine:

cat /proc/buddyinfo

The output shows one row per memory zone, with a column for every order from 0 up to the architecture’s maximum order. Each number tells you how many free, physically contiguous blocks currently exist at that order. A high count at low orders combined with very low counts at high orders is a classic early sign of external fragmentation — something embedded and driver developers should keep an eye on over long-running systems.

Using the Page Allocator APIs in Kernel Code

The kernel exposes the buddy allocator to module and driver writers through a small family of allocation and free functions, declared in linux/gfp.h and linux/mm.h. The most commonly used pair in current kernels are alloc_pages() / __get_free_pages() for allocation, and __free_pages() for releasing memory back to the allocator.

#include <linux/gfp.h>
#include <linux/mm.h>

/* Allocate 2^order contiguous pages */
unsigned long addr = __get_free_pages(GFP_KERNEL, order);
if (!addr) {
    /* handle allocation failure */
}

/* ... use the memory ... */

/* Release it back to the buddy allocator */
free_pages(addr, order);

The first argument, the GFP flag, tells the allocator the context and behavior expected of the request — for example, GFP_KERNEL is used for normal allocations that are allowed to sleep, while GFP_ATOMIC is used inside interrupt context where the call must not sleep. Choosing the correct GFP flag is one of the most important practical skills in kernel and driver development, and it is something every free Linux device drivers course should cover in depth.

Real-World Use Cases

Common Mistakes and Troubleshooting Tips

Best Practices

Performance Considerations

Because the buddy allocator coalesces and splits blocks using simple bitwise and pointer operations, allocation and free operations are fast — typically O(log N) relative to the order requested. However, high-order allocations become progressively more expensive and more likely to fail as system uptime increases and fragmentation grows, which is why drivers should minimize reliance on large contiguous allocations wherever possible.

Security Considerations

Kernel memory handed out by the page allocator is never automatically zeroed unless the caller requests it (for example via __GFP_ZERO). Drivers that allocate pages destined to be shared with user space or hardware must zero or otherwise sanitize the memory before exposing it, to avoid leaking stale kernel data.

Summary / Key Takeaways

Conclusion

The Linux kernel page allocator may sit several layers below the APIs most driver developers use day to day, but understanding the buddy system is what turns a driver writer who copies code into one who can genuinely debug memory-related failures, fragmentation issues, and DMA allocation problems. This lesson is part of our ongoing free Linux kernel development course and free Linux device drivers course at EmbeddedPathashala — continue to the next lecture to go deeper into kernel memory management.

Frequently Asked Questions