The Linux Kernel Module Programming Guide (LKMPG) is a comprehensive, community-maintained guide to writing Linux kernel modules. It consists of a LaTeX book and a large collection of example kernel modules that demonstrate a wide range of kernel APIs and programming patterns, including character devices, the /proc filesystem, sysfs, debugfs, system call interception, synchronization primitives (mutex, spinlock, rwlock, RCU), interrupt handling, timers, workqueues, block devices, network drivers, DMA, device tree, GPIO, and more. The repository also includes a QEMU-based development environment (devtools) that automates building a minimal kernel and initramfs, allowing developers to compile and test modules inside a virtual machine without affecting their host system. The code is well-structured, follows kernel coding conventions, and includes proper error handling and cleanup. Many examples include version guards for compatibility across different kernel versions (5.10 through 6.17+). The devtools scripts are robust, with caching, idempotency, and prebuilt binary support to speed up setup. The CI pipeline builds the book, compiles all examples, runs static analysis (cppcheck, sparse, gcc, smatch), and tests module loading/unloading in both bare-metal and QEMU environments. While primarily educational, the example modules are self-contained and can serve as templates for real kernel module development. The devtools provide a reproducible environment for kernel module testing, which is valuable for developers working on out-of-tree modules. The repository is actively maintained and targets recent Linux 5.x and 6.x kernels.

simplefs is a Linux kernel module that implements a simple extent-based filesystem for educational use, accompanied by a userspace mkfs.simplefs formatter. The kernel component registers a VFS filesystem type and implements superblock, inode, file, and directory operations. The on-disk layout is a fixed 4 KiB block format: superblock, inode store, inode free bitmap, block free bitmap, and data blocks. Inodes are 72 bytes and point to an extent-index block; regular files use extents of up to 8 contiguous blocks (maximum file size around 10.65 MiB), while directories use a hashed ring of extents to support up to 40,920 entries. Functionally, the filesystem supports create, lookup, unlink, rename, hard links, symbolic links, mkdir, rmdir, and read/write. File I/O is implemented through both traditional VFS page-cache helpers and simpler direct buffer-head read/write paths, making the code a useful teaching reference for how Linux VFS callbacks and buffer management interact. External journaling is supported via the kernel's jbd2 subsystem, mounted with a journal_path or journal_dev option, though only extent metadata is journaled and the journal size is currently hardcoded. Entry points are module_init/module_exit in fs.c, the mount path through simplefs_fill_super in super.c, and main() in mkfs.c. Dependencies are Linux kernel headers, jbd2, block-device APIs, and a Linux or macOS host for mkfs. The repository includes shell-based integration tests that exercise maximum directory fill, large-file behavior, remount persistence, random create/delete, and free-block reclamation, plus CI jobs for formatting and macOS mkfs verification. The code maintains compatibility macros spanning Linux 5.10 through 7.3, reflecting active upkeep.