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.