Day93 - vmalloc() Internals¶

Objective¶

Learn how Linux vmalloc() provides virtually contiguous memory by mapping multiple independent physical pages into a continuous virtual address range.

In this lab, we build a simplified vmalloc() simulator on top of the Buddy Allocator and compare its behavior with the real Linux kernel implementation.

Learning Objectives¶

After completing this lab, you should understand:

Why vmalloc() exists
Difference between kmalloc() and vmalloc()
Virtually contiguous vs physically contiguous memory
Virtual address management
Physical page mapping
Address translation with vmalloc_to_page()
Relationship between vmalloc() and the Buddy Allocator

Lab Architecture¶

                vmalloc()
                     │
                     ▼
        +----------------------+
        |   vmalloc_area       |
        +----------------------+
        | Virtual Address      |
        | Size                 |
        | struct page *[]      |
        +----------------------+
                     │
                     ▼
             Buddy Allocator
                     │
                     ▼
              Physical Pages

Unlike kmalloc(), each mapped page may come from a different physical location.

Lab 1 - Basic Allocation and Free¶

Goal¶

Understand the lifecycle of a vmalloc allocation.

Verify¶

vmalloc()
vfree()
Virtual address allocation
Page rounding
Area insertion/removal

Example:

vmalloc(1)

↓

1 page allocated

↓

Virtual Address
0x10000000

Lab 2 - Virtual Address Translation¶

Goal¶

Implement and verify vmalloc_to_page().

The translation flow is:

Virtual Address
        │
        ▼
Find vmalloc area
        │
        ▼
Calculate page index
        │
        ▼
struct page
        │
        ▼
PFN

Verify:

First page
Second page
Page boundary
Invalid addresses
Translation after vfree()

Lab 3 - Physically Non-contiguous Pages¶

Goal¶

Demonstrate that contiguous virtual memory does not require contiguous physical memory.

The lab intentionally fragments physical memory before calling vmalloc().

Example:

Virtual Range

0x10000000
0x10001000

↓

PFN 1
PFN 3

Although the PFNs are not contiguous, the virtual address range remains continuous.

Lab 4 - Linux Kernel Verification¶

Goal¶

Compare the simulator with the real Linux kernel.

Kernel APIs:

vmalloc()
vmalloc_to_page()
page_to_pfn()
vfree()

Typical output:

Virtual Range

ffffc000825b8000
ffffc000825bc000
ffffc000825c0000
ffffc000825c4000

PFNs

229847
2728
33491
66084

The virtual addresses increase by one PAGE_SIZE, while the PFNs are not necessarily contiguous.

Note

The simulator uses a 4 KiB page size, while the Raspberry Pi 5 kernel used in this lab is configured with a 16 KiB page size.

Although the page size differs, both implementations follow the same address translation principle.

Key Takeaways¶

kmalloc() provides physically contiguous memory.
vmalloc() provides virtually contiguous memory.
vmalloc() allocates pages individually from the Buddy Allocator.
Virtual addresses remain contiguous even when physical pages are scattered.
vmalloc_to_page() translates a virtual address into its backing physical page.