kernel-memory-management

Kernel Memory Management

Purpose

Guide agents through Linux kernel memory allocation: physical page allocator (buddy), SLUB kmalloc caches, vmalloc virtual mappings, zones (DMA/Normal), and safe allocation patterns in drivers — extending skills/kernel/kernel-internals.

When to Use

• Choosing kmalloc vs vmalloc vs get_free_pages
• Debugging kernel OOM or slab corruption
• DMA buffer allocation requirements
• Understanding /proc/meminfo and slab stats

Workflow

1. Allocator decision tree

Need physically contiguous memory for DMA?
├── Yes → dma_alloc_coherent() or CMA pool
└── No
    ├── Size ≤ ~128K (arch-dependent) → kmalloc / kzalloc
    ├── Large or non-contiguous OK → vmalloc / vzalloc
    └── Whole pages → alloc_pages() / __get_free_pages()

2. kmalloc and SLUB

SLUB is the default general-purpose kmalloc backend (per kernel memory allocation guide); the legacy SLAB allocator was removed in Linux 6.12.

void *buf = kmalloc(size, GFP_KERNEL);
if (!buf)
    return -ENOMEM;
/* ... */
kfree(buf);

GFP flag	Context
GFP_KERNEL	Process context, may sleep
GFP_ATOMIC	IRQ / holding spinlock — smaller pools
GFP_DMA	DMA zone (legacy 32-bit devices)

Check /proc/slabinfo and cat /sys/kernel/slab/*/object_size.

3. vmalloc

void *v = vmalloc(size);  /* virtually contiguous, physically scattered */
vfree(v);

Use for large buffers where contiguous physical pages are unnecessary. Do not use for DMA without dma_map_*.

4. Buddy page allocator

struct page *page = alloc_pages(GFP_KERNEL, order);
void *addr = page_address(page);
__free_pages(page, order);

order n allocates 2^n pages. Zone selection: ZONE_DMA, ZONE_NORMAL, ZONE_MOVABLE (see include/linux/mmzone.h).

5. NUMA basics

On NUMA systems, kmalloc prefers local node; use alloc_pages_node() or mbind policies for HPC paths — see skills/allocators/numa-programming.

6. Debugging

cat /proc/meminfo
cat /proc/slabinfo
dmesg | grep -i oom

Kernel config: CONFIG_SLUB_DEBUG, CONFIG_KASAN for use-after-free.

7. Agent usage

/kernel-memory-management Choose allocator for 2 MiB driver ring buffer in process context

Common Problems

Symptom	Cause	Fix
kmalloc fails large	Exceeds kmalloc limit	Use vmalloc or pages
DMA buffer wrong	Used vmalloc for HW DMA	dma_alloc_coherent
OOM killer	Unbounded cache growth	Limit pools; use shrinker
Slab corruption	Use-after-free	KASAN; audit kfree pairing
GFP_KERNEL in IRQ	Sleeping allocator in atomic	GFP_ATOMIC

Related Skills

• skills/kernel/kernel-internals — page cache, OOM killer
• skills/kernel/device-drivers — dma_alloc_coherent
• skills/allocators/custom-allocators — userspace allocator design
• skills/allocators/numa-programming — NUMA-aware allocation
• skills/runtimes/sanitizers — KASAN overlap