Part 5: Memory Management

Memory management in the kernel is fundamentally different from user space. You have direct access to physical memory, must manage allocation contexts carefully, and need to understand DMA for hardware interaction.

Memory Landscape

flowchart TB
    subgraph Allocators["Memory Allocators"]
        kmalloc["kmalloc()<br/>Small objects, physically contiguous"]
        vmalloc["vmalloc()<br/>Large objects, virtually contiguous"]
        slab["kmem_cache<br/>Object caches"]
        percpu["Per-CPU<br/>No locking needed"]
    end

    subgraph DMA["DMA Memory"]
        coherent["Coherent DMA<br/>dma_alloc_coherent()"]
        streaming["Streaming DMA<br/>dma_map_single()"]
    end

    subgraph MMIO["Memory-Mapped I/O"]
        ioremap["ioremap()<br/>Device registers"]
        mmap["mmap()<br/>User space mapping"]
    end

    style Allocators fill:#738f99,stroke:#0277bd
    style DMA fill:#8f8a73,stroke:#f9a825
    style MMIO fill:#8f7392,stroke:#6a1b9a

Chapter Contents

Chapter	Topic	Key Concepts
5.1	Memory Allocators	kmalloc, kzalloc, vmalloc, kvmalloc
5.2	GFP Flags	Allocation context, GFP_KERNEL vs GFP_ATOMIC
5.3	Slab Allocator	kmem_cache, object pools
5.4	Per-CPU Variables	Lock-free per-CPU data
5.5	Coherent DMA	dma_alloc_coherent, consistent mappings
5.6	Streaming DMA	dma_map_single, buffer synchronization
5.7	Memory-Mapped I/O	ioremap, device registers
5.8	mmap	Mapping memory to user space

Quick Reference: Choosing an Allocator

flowchart TD
    Start["Need memory?"]
    Size{"Size?"}
    Contig{"Need physically<br/>contiguous?"}
    Sleep{"Can sleep?"}
    Freq{"Frequent alloc/free<br/>of same size?"}

    Start --> Size
    Size -->|"< PAGE_SIZE"| Freq
    Freq -->|Yes| Slab["kmem_cache"]
    Freq -->|No| Small["kmalloc()"]
    Size -->|"≥ PAGE_SIZE"| Contig
    Contig -->|Yes| Pages["alloc_pages()<br/>or kmalloc()"]
    Contig -->|No| Large["vmalloc()"]
    Sleep -->|No| Atomic["GFP_ATOMIC"]
    Sleep -->|Yes| Kernel["GFP_KERNEL"]

    style Slab fill:#7a8f73,stroke:#2e7d32
    style Small fill:#738f99,stroke:#0277bd
    style Large fill:#8f8a73,stroke:#f9a825

Allocator	Size Range	Contiguous	Can Sleep	Use Case
`kmalloc()`	< ~128KB	Physical	With GFP_KERNEL	General small allocations
`vmalloc()`	> PAGE_SIZE	Virtual only	Yes	Large allocations
`kvmalloc()`	Any	Best effort	Yes	Flexible sizing
`kmem_cache`	Fixed	Physical	Depends	Frequent same-size alloc
`alloc_pages()`	Pages	Physical	Depends	Page-aligned buffers

Examples

This part includes working examples:

kmem-cache-demo: Custom slab cache usage
dma-demo: DMA buffer mapping

Key Differences from User Space

Aspect	User Space	Kernel Space
Allocation failure	Returns NULL, can retry	May panic or return NULL
Sleeping	Always OK	Depends on context
Virtual memory	Unified address space	Split kernel/user
Page faults	Handled automatically	Can crash system
Memory limits	Per-process	System-wide

Prerequisites

Before starting this part, ensure you understand:

Execution contexts (Part 2)
Synchronization basics (Part 4)
Module lifecycle (Part 2)