Subsystems

The Linux kernel is organized as a single binary built from many cooperating subsystems. This wiki uses the top-level source directories under the repository root as the unit of organization. Each page below maps one (or a closely related cluster) of those directories to its purpose, key files, and entry points for modification.

Subsystem map

graph TD
    INIT["init/<br/>Boot, start_kernel"]
    KCORE["kernel/<br/>Process, locking, RCU, BPF, cgroup"]
    SCHED["kernel/sched/<br/>Scheduler"]
    MM["mm/<br/>Memory management"]
    FS["fs/<br/>VFS + filesystems"]
    BLOCK["block/<br/>Block layer + blk-mq"]
    IOU["io_uring/<br/>Async I/O"]
    NET["net/<br/>Network stack"]
    DRV["drivers/<br/>Device drivers"]
    ARCH["arch/<br/>Per-architecture"]
    SEC["security/<br/>LSMs"]
    CRYPTO["crypto/<br/>Crypto API"]
    IPC["ipc/<br/>SysV IPC"]
    SOUND["sound/<br/>ALSA"]
    VIRT["virt/<br/>KVM common"]
    RUST["rust/<br/>Rust support"]
    LIB["lib/<br/>Helpers"]
    INC["include/<br/>Headers"]
    TOOL["tools/<br/>perf, bpftool, selftests"]
    SCRIPT["scripts/<br/>Build helpers"]
    SAMPLES["samples/<br/>Example modules"]
    USR["usr/<br/>Initramfs"]
    CERT["certs/<br/>Module signing"]

    INIT --> KCORE
    KCORE --> SCHED
    KCORE --> MM
    MM --> FS
    MM --> BLOCK
    BLOCK --> FS
    FS --> IOU
    NET -.uses.-> KCORE
    DRV -.uses.-> KCORE
    DRV -.uses.-> MM
    ARCH --> KCORE
    SEC -.hooks into.-> FS
    SEC -.hooks into.-> NET
    CRYPTO -.used by.-> NET
    CRYPTO -.used by.-> FS
    SOUND -.uses.-> DRV
    VIRT -.uses.-> KCORE
    RUST -.linked into.-> KCORE

Pages

Subsystem	Source	Page
Boot path	init/	Init
Kernel core	kernel/	Kernel core
Scheduler	kernel/sched/	Scheduler
Memory	mm/	Memory management
Filesystems	fs/	Filesystems
Block layer	block/	Block layer
io_uring	io_uring/	io_uring
Networking	net/	Networking
Drivers	drivers/	Drivers
Architecture support	arch/	Architecture support
Security / LSM	security/	Security
Crypto	crypto/	Crypto
IPC	ipc/	IPC
Sound (ALSA)	sound/	Sound
Virtualization (KVM)	virt/	Virtualization
Rust support	rust/	Rust support
Library helpers	lib/	Lib
Headers	include/	Include
Tools	tools/	Tools
Build scripts	scripts/	Scripts
Samples	samples/	Samples
Initramfs generator	usr/	usr
Module signing	certs/	Certs

How to read these pages

Each subsystem page follows the same shape:

1. Purpose — what the directory exists for.
2. Directory layout — the most useful files and subdirectories.
3. Key abstractions — the core data types and APIs.
4. How it works — main flows, with diagrams where helpful.
5. Integration points — what it imports and what it exports.
6. Entry points for modification — where to start if you want to change something.

If you want a guided tour, start with Kernel core, Memory management, and Filesystems in that order. Those three pages cover the heart of the kernel; the rest of the tree builds on them.