The document describes the Linux Virtual File System (VFS) which provides a standard interface for file access across different file systems by handling file system calls and interacting with individual file systems. It discusses the key data structures in VFS including inodes, dentries, superblocks and the methods involved for operations like lookups, permission checks, attribute retrieval. The VFS acts as an abstraction layer that allows file operations to work consistently regardless of the underlying file system type.

1. Linux Virtual File System Peter J. Braam

6. Linux is Obsolete Andrew Tanenbaum Usefulness

11. Anatomy of stat system call sys_stat(path, buf) { dentry = namei(path); if ( dentry == NULL ) return -ENOENT; inode = dentry->d_inode; rc =inode->i_op->i_permission(inode); if ( rc ) return -EPERM; rc = inode->i_op->i_getattr(inode, buf); dput(dentry); return rc; } Establish VFS data Call into inode layer of filesystem Call into inode layer of filesystem

12. Anatomy of fstatfs system call sys_fstatfs(fd, buf) { /* for things like “df” */ file = fget(fd); if ( file == NULL ) return -EBADF; superb = file->f_dentry->d_inode->i_super; rc = superb->sb_op->sb_statfs(sb, buf); return rc; } Call into superblock layer of filesystem Translate fd to VFS data structure

15. namei struct dentry *namei(parent, name) { if (dentry = d_lookup(parent,name)) else ddd_hash(parent, name) ddd_revalidate(dentry) iii_lookup(parent, name) sss_read_inode(…) struct inode *iget(ino, dev) { /* try cache else .. */ } VFS FS

19. What’s inside an inode - 1 list_head i_hash list_head i_list list_head i_dentry int i_count long i_ino int i_dev {m,a,c}time {u,g}id mode size n_link caching Identifies file Usual stuff

20. What’s inside an inode -2 superblock i_sb inode_ops i_op wait objects, semaphore lock vm_area_struct pipe/socket info page information union { ext2fs_inode_info i_ext2 nfs_inode_info i_nfs coda_inode_info i_coda ..} u Which FS For mmap, networking waiting FS Specific info: blockno’s fids etc

22. Inode Cache Dirty inodes Inode_hashtable 1. iget: if i_count>0 ++ 2. iput: if i_count>1 - - sss_write_inode (sync one) Used inodes Unused inodes sss_read_inode (iget) sss_clear_inode (freeing inos) or sss_delete_inode (iput) media fs only (mark_inode_dirty) 3. free_inodes 4. syncing inodes Players: Fs storage Fs storage Fs storage

23. Sales Red Hat Software sold 240,000 copies of Red Hat Linux in 1997 and expects to reach 400,000 in 1998. Estimates of installed servers (InfoWorld): - Linux: 7 million - OS/2: 5 million - Macintosh: 1 million

27. Dentry associated lists d_alias chains place : d_instantiate remove : dentry_iput inode I_dentry list head d_child chains place : d_alloc remove : d_prune, d_invalidate, d_put inode i_dentry list head = d_inode pointer = d_parent pointer dentry inode relationship dentry tree relationship Legend: inode dentry

31. Style Dijkstra probably hates me Linus Torvalds