Linux Virtual File System

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Linux Virtual File System

• Peter J. Braam

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Aims

• Present the data structures in Linux VFS
• Provide information about flow of control
• Describe methods and invariants needed to
  implement a new file system
• Illustrate with some examples

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History
File access

• BSD implemented VFS for NFS aim dispatch to
  different filesystems
• VMS had elaborate filesystem
• NT/Win95 have VFS type interfaces
• Newer systems integrate VM with buffer cache.

VFS
nfs
ufs
Coda
disk
Venus
udp
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Linux Filesystems

• Media based
• ext2 - Linux native
• ufs - BSD
• fat - DOS FS
• vfat - win 95
• hpfs - OS/2
• minix - well.
• Isofs - CDROM
• sysv - Sysv Unix
• hfs - Macintosh
• affs - Amiga Fast FS
• NTFS - NTs FS
• adfs - Acorn-strongarm

• Network
• nfs
• Coda
• AFS - Andrew FS
• smbfs - LanManager
• ncpfs - Novell
• Special ones
• procfs -/proc
• umsdos - Unix in DOS
• userfs - redirector to user

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Linux Filesystems (ctd)

• Forthcoming
• devfs - device file system
• DFS - DCE distributed FS
• Varia
• cfs - crypt filesystem
• cfs - cache filesystem
• ftpfs - ftp filesystem
• mailfs - mail filesystem
• pgfs - Postgres versioning file system

• Linux serves (unrelated to the VFS!)
• NFS - user kernel
• Coda
• AppleShare - netatalk/CAP
• SMB - samba
• NCP - Novell

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Linux is Obsolete
Usefulness

• Andrew Tanenbaum

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Linux VFS

• Multiple interfaces build up VFS
• files
• dentries
• inodes
• superblock
• quota
• VFS can do all caching provides utility fctns
  to FS
• FS provides methods to VFS many are optional

File access
VFS
nfs
VFS
ext2fs
Coda FS
VFS
disk
udp
Venus
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User level file access

• Typical user level types and code
• pathnames /myfile
• file descriptors fd open(/myfile)
• attributes in struct stat stat(/myfile,
  mybuf), chmod, chown...
• offsets write, read, lseek
• directory handles DIR dh opendir(/mydir)
• directory entries struct dirent ent
  readdir(dh)

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VFS

• Manages kernel level file abstractions in one
  format for all file systems
• Receives system call requests from user level
  (e.g. write, open, stat, link)
• Interacts with a specific file system based on
  mount point traversal
• Receives requests from other parts of the kernel,
  mostly from memory management

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File system level

• Individual File Systems
• responsible for managing file directory data
• responsible for managing meta-data timestamps,
  owners, protection etc
• translates data between
• particular FS data e.g. disk data, NFS data,
  Coda/AFS data
• VFS data attributes etc in standard format
• e.g. nfs_getattr(.) returns attributes in VFS
  format, acquires attributes in NFS format to do
  so.

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Anatomy of stat system call
sys_stat(path, buf) dentry namei(path)
if ( dentry NULL ) return -ENOENT inode
dentry-gtd_inode rc inode-gti_op-gti_permission(i
node) if ( rc ) return -EPERM rc
inode-gti_op-gti_getattr(inode, buf)
dput(dentry) return rc
Establish VFS data
Call into inode layer of filesystem
Call into inode layer of filesystem
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Anatomy of fstatfs system call
sys_fstatfs(fd, buf) / for things
like df / file fget(fd) if ( file
NULL ) return -EBADF superb
file-gtf_dentry-gtd_inode-gti_super rc
superb-gtsb_op-gtsb_statfs(sb, buf) return rc
Translate fd to VFS data structure
Call into superblock layer of filesystem
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Data structures

• VFS data structures for
• VFS handle to the file inode (BSD vnode)
• User instantiated file handle file (BSD file)
• The whole filesystem superblock (BSD vfs)
• A name to inode translation dentry

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Shorthand method notation

• super block methods sss_methodname
• inode methods iii_methodname
• dentry methods ddd_methodname
• file methods fff_methodname
• instead of
• inode i_op lookup we write iii_lookup

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namei
FS
VFS
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 .. /
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Superblocks

• Handle metadata only (attributes etc)
• Responsible for retrieving and storing metadata
  from the FS media or peers
• Struct superblocks hold things like
• device, blocksize, dirty flags, list of dirty
  inodes
• super operations
• wait queue
• pointer to the root inode of this FS

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Super Operations (sss_)

• Ops on Inodes
• read_inode
• put_inode
• write_inode
• delete_inode
• clear_inode
• notify_change

• Superblock manips
• read_super (mount)
• put_super (unmount)
• write_super (unmount)
• statfs (attributes)

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Inodes

• Inodes are VFS abstraction for the file
• Inode has operations (iii_methods)
• VFS maintains an inode cache, NOT the individual
  FSs (compare NT, BSD etc)
• Inodes contain an FS specific area where
• ext2 stores disk block numbers etc
• AFS would store the FID
• Extraordinary inode ops are good for dealing with
  stale NFS file handles etc.

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Whats 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,ctime u,gid mode size n_link
caching
Identifies file
Usual stuff
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Whats 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 blocknos fids etc
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Inode state

• Inode can be on one or two lists
• (hash in_use) or (hash dirty ) or unused
• inode has a use count i_count
• Transitions
• unused ? hash iget calls sss_read_inode
• dirty? in_use sss_write_inode
• hash ? unused call on sss_clear_inode, but if
• i_nlink 0 iput calls sss_delete_inode when
  i_count falls to 0

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Inode Cache
1. iget if i_countgt0 2. iput if i_countgt1 - -
3. free_inodes 4. syncing inodes
Players
Inode_hashtable
sss_clear_inode (freeing inos) or sss_delete_inode
(iput)
sss_read_inode (iget)
Unused inodes
Dirty inodes
sss_write_inode (sync one)
media fs only (mark_inode_dirty)
Used inodes
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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
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Inode operations (iii_)

• symbolic links
• readlink
• follow link
• pages
• readpage, writepage, updatepage - read or write
  page. Generic for mediafs.
• bmap - return disk block number of logical block
• special operations
• revalidate - see dentry sect
• truncate
• permission

• lookup return inode
• calls iget
• creation/removal
• create
• link
• unlink
• symlink
• mkdir
• rmdir
• mknod
• rename

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Dentry world

• Dentry is a name to inode translation structure
• Cached agressively by VFS
• Eliminates lookups by FS private caches
• timing on Coda FS ls -lR 1000 files after
  priming cache
• linux 2.0.32 7.2secs
• linux 2.1.92 0.6secs
• disk fs less benefit, NFS even more
• Negative entries!
• Namei is dramatically simplified

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Inside dentrys

• name
• pointer to inode
• pointer to parent dentry
• list head of children
• chains for lots of lists
• use count

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Dentry associated lists
Legend
inode
dentry
dentry inode relationship
dentry tree relationship
inode I_dentry list head
inode i_dentry list head
d_inode pointer
d_parent pointer
d_child chains place d_alloc remove d_prune,
d_invalidate, d_put
d_alias chains place d_instantiate remove
dentry_iput
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Dcache
dentry_hashtable (d_hash chains)

• namei tries cache d_lookup
• ddd_compare
• Success ddd_revalidate
• d_invalidate if fails
• proceed if success
• Failure iii_lookup
• find inode
• iget
• sss_read_inode
• finish
• d_add
• can give negative entry in dcache

dhash(parent, name) list head
namei iii_lookup d_add
prune d_invalidate d_drop
unused dentries (d_lru chains)
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Dentry methods

• ddd_revalidate can force new lookup
• ddd_hash compute hash value of name
• ddd_compare are names equal?
• ddd_delete, ddd_put, ddd_iput FS cleanup
  opportunity

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Dentry particulars

• ddd_hash and ddd_compare have to deal with
  extraordinary cases for msdos/vfat
• case insensitive
• long and short filename pleasantries
• ddd_revalidate -- can force new lookup if inode
  not in use
• used for NFS/SMBfs aging
• used for Coda/AFS callbacks

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Style
Dijkstra probably hates me Linus Torvalds
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Memory mapping

• vm_area structure has
• vm_operations
• inode, addresses etc.
• vm_operations
• map, unmap
• swapin, swapout
• nopage -- read when page isnt in VM
• mmap
• calls on iii_readpage
• keeps a use count on the inode until unmap