Linux source code tour

Version: 2.6.17.13

Source tree

• root of Linux source code tree
• arch/ directories - contain architecture-specific source files
• include/ directory - contains public includes
• include/asm-xxx/ directories - contain architecture-specific include files and inline code
• Documentation/ directory and scripts/ directory - auxiliary things
• other directories contain code for various subsystems 

Functional areas

• User process management (CPU scheduling, task accounting, forking, etc.) - kernel/ directory
• IPC services for processes (pipes, shared memory, etc.) - ipc/ directory
• Virtual memory management - mm/ directory
• Low-level hardware support (buses, interrupts, CPUs, hotplug, driver model, power management) - drivers/base/ directory, kernel/irq/ directory, arch/i386/kernel/cpu/ directory, kernel/power/ directory 
• Device drivers - drivers/ directory, sound/ directory
• Networking (sockets handling, network stacks) - net/ directory
• Disks, block devices and volume management - block/ directory, drivers/block/ directory, drivers/md/ directory, files in fs/ directory
• Character devices - drivers/char/ directory
• Filesystems -  files in fs/ directory (generic layer) and subdirectories with specific filesystems implementation
• Security - security/ directory
• Time management - kernel/ directory
• Locks, compression, cryptography and other library-like services for kernel code -  lib/ directory, crypto/ directory

User process management

• struct task_struct -  contains all information about process (or thread), e.g.:
• pid
• relationships (parent/child, group, ...)
• thread information
• security privileges (effective user id, SELinux context, etc.)
• file descriptors
• process virtual memory description (struct mm_struct)
• scheduling information (state, slice used, sleep time, allowed CPUs, etc.)
• process state information (signals, tracing, etc.)
• CPU context (struct thread_struct)
• ...
• struct files_struct, struct fdtable - file descriptors table structures
• struct file - description of file usage by process

Virtual memory management

• struct mm_struct - process virtual memory description, shared between threads (as they share virtual memory)
• struct vm_area_struct *mmap - list of VM areas
• pgd_t *pgd - physical page table address
• struct vm_area_struct - description of one virtual memory area (i.e. contigous space with the same handler, mmapped file, code area, stack, etc.)
• struct vm_operations_struct - operations on VM area (handling page faults, memory protection, etc.)

• struct page - describes one physical page of memory
• struct address_space *mapping - description for pages mmapped from file
• pgoff_t index - index in "mapping"
• struct mmu_gather - TLB context
• struct pte_t - page table entry (physical, CPU specific)
• struct pmd_t - 2-level page directory
• struct pgd_t - 3-level page directory
• physical level page manipulation functions (i386)

• struct swp_entry_t - description of page which is in swap (in struct page)
• struct swap_info_struct - description of one swap area

• struct pgoff_t - index in page cache file mapping
• struct address_space - describes mapping of a file to page cache
• struct address_space_operations - operations which can be performed on file pages mapped to page cache

Low-level hardware support

• PnP support
• Driver model (and sysfs)
• Power management
• Buses
• PCI
• USB
• PCMCIA
• ...
• IRQ management
• DMA management
• CPU hotplug
• Physical memory
• ...

• struct bus_type - bus representation
• struct device - device representation
• struct bus_type * bus - bus on which the device exists
• struct device_driver *driver - which driver has allocated this device
• struct dev_pm_info power - power state information
• struct device_driver - driver
• int     (*remove)       (struct device * dev) - remove device
• void    (*shutdown)     (struct device * dev) - turn off device
• int     (*suspend)      (struct device * dev, pm_message_t state) - put device into low-power mode
• int     (*resume)       (struct device * dev) - wake up device from low-power mode
• struct resource - resource representation

• Power management definitions
• struct dev_pm_info - device power state

• struct pci_bus - PCI bus structure, contains operations specific for PCI bus
• struct pci_dev - PCI device specific representation
• struct  device dev - driver model device
• struct pci_bus  *bus - PCI bus where this device belongs
• struct usb_bus - similar for USB bus
• ...

• request_irq() - assigns action to IRQ and enables it
• struct irqaction - defines action run when IRQ happens
• struct irqaction *next - next in chain of IRQ (for shared IRQs)
• handler - function to be called

Networking

• sockets handling
• network protocols
• IP
• IPX
• NetBEUI
• ...
• physical network stacks
• Ethernet
• PPP
• ...
• packet scheduling
• encapsulation
• ...

• struct sock - representation of network socket
• struct sk_buff *head - first socket buffer for this socket
• struct inet_sock - representation of internet socket type
• struct sock sk - generic socket part
• struct inet_connection_sock - connection-oriented socket
• struct inet_sock icsk_inet - internet socket part
• struct tcp_sock - TCP socket
• struct sk_buff - buffer for packet
• struct sock *sk - socket
• struct net_device *dev - network device
• struct tcphdr *th - TCP header
• struct iphdr *iph - IP header

• struct net_device - physical network interface description and operations

Disks, block devices and volume management

• Disk devices handling (I/O requests, scheduling)
• Block devices
• Physical disks and arrays
• Virtual block devices (DM, MD, LVM, multipath, NBD)
• Partition management
• Logical volumes management
• Page cache integration

• struct block_device - block device information
• struct gendisk *bd_disk
• struct gendisk - physical disk representation
• struct hd_struct **part - partitions
• struct block_device_operations *fops - disk operations
• struct request_queue *queue - I/O requests queue
• struct block_device_operations - operations performed on block device (open, ioctl, media change, get geometry)
• struct hd_struct - partition information
• struct request_queue - queue of I/O requests to disk device
• elevator_t *elevator - I/O scheduling algorithm
• requests queue operations (insert, merge, flush)
• struct request - represents I/O request
• struct bio *bio - head of bios queue for this request
• unsigned short ioprio - I/O priority
• elevator fields
• struct bio - basic unit of I/O on physical level, can hold scatter/gather information, allows merging of requests on physical operation level or processing it in hardware, if it is capable
• struct elevator_queue - description of I/O scheduler
• elevator_ops - operations of I/O scheduler

• struct buffer_head - representation of disk block in page cache
• struct page *b_page - page where this block is mapped
• sector_t b_blocknr - block number on disk
• struct block_device *b_bdev - block device of disk
• struct backing_dev_info - description and operations of deviced used to hold pages

Character devices

• struct file_operations - operations on file which represents character device

Filesystems

• Generic VFS layer
• Inodes handling 
• Directory handling
• File handling
• File caching and readahead (integration with page cache)
• Specific filesystems implementations

• struct file - representation of file (in process)
• const struct file_operations *f_op - operations on file
• struct dentry *f_dentry - directory entry
• struct vfsmount *f_vfsmnt -  mount point of filesystem where file resides
• struct file_ra_state f_ra - file readahead state
• struct address_space *f_mapping - description of caching file contents on backing device (see VM)
• struct file_operations - generic interface of operations on file
• struct dentry - representation of directory entry (i.e. name)
• struct inode *d_inode - inode
• struct dentry_operations *d_op - operations
• struct dentry_operations - operations on file name
• struct inode - inode representation in memory
• struct inode_operations *i_op - inode operations
• struct block_device *i_bdev - block device
• struct inode_operations - operations on inode
• create file, mkdir, lookup, rename, unlink, setattr
• struct vfsmount - filesystem mount description
• struct super_block *mnt_sb
• struct super_block - description of filesystem
• struct super_operations *s_op
• struct super_operations - operations on filesystem
• alloc_inode
• read_inode
• write_inode
• statfs
• write_super
• struct file_system_type - registered filesystem type

Other important files

• Documentation/ directory - documentation for kernel (interfaces documentation, implementation documentation, submissions procedures, etc.)
• Coding style guide
• Patch submission process
• scripts/ directory - useful scripts for kernel development (patches, generating necessary files, config stuff)
• Makefile files
• Kbuild files - kernel build configuration files

Further reading

• Linux device drivers book (online)
• Linux MM docs
• Daniel P. Bovet; Marco Cesati - Understanding the Linux Kernel, 3rd Edition