mohitmishra786/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