Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 1 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 2 | Overview of the Linux Virtual File System |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 3 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 4 | Original author: Richard Gooch <rgooch@atnf.csiro.au> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 5 | |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 6 | Last updated on June 24, 2007. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 7 | |
| 8 | Copyright (C) 1999 Richard Gooch |
| 9 | Copyright (C) 2005 Pekka Enberg |
| 10 | |
| 11 | This file is released under the GPLv2. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 12 | |
| 13 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 14 | Introduction |
| 15 | ============ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 16 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 17 | The Virtual File System (also known as the Virtual Filesystem Switch) |
| 18 | is the software layer in the kernel that provides the filesystem |
| 19 | interface to userspace programs. It also provides an abstraction |
| 20 | within the kernel which allows different filesystem implementations to |
| 21 | coexist. |
| 22 | |
| 23 | VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so |
| 24 | on are called from a process context. Filesystem locking is described |
| 25 | in the document Documentation/filesystems/Locking. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 26 | |
| 27 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 28 | Directory Entry Cache (dcache) |
| 29 | ------------------------------ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 30 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 31 | The VFS implements the open(2), stat(2), chmod(2), and similar system |
| 32 | calls. The pathname argument that is passed to them is used by the VFS |
| 33 | to search through the directory entry cache (also known as the dentry |
| 34 | cache or dcache). This provides a very fast look-up mechanism to |
| 35 | translate a pathname (filename) into a specific dentry. Dentries live |
| 36 | in RAM and are never saved to disc: they exist only for performance. |
| 37 | |
| 38 | The dentry cache is meant to be a view into your entire filespace. As |
| 39 | most computers cannot fit all dentries in the RAM at the same time, |
| 40 | some bits of the cache are missing. In order to resolve your pathname |
| 41 | into a dentry, the VFS may have to resort to creating dentries along |
| 42 | the way, and then loading the inode. This is done by looking up the |
| 43 | inode. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 44 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 45 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 46 | The Inode Object |
| 47 | ---------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 48 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 49 | An individual dentry usually has a pointer to an inode. Inodes are |
| 50 | filesystem objects such as regular files, directories, FIFOs and other |
| 51 | beasts. They live either on the disc (for block device filesystems) |
| 52 | or in the memory (for pseudo filesystems). Inodes that live on the |
| 53 | disc are copied into the memory when required and changes to the inode |
| 54 | are written back to disc. A single inode can be pointed to by multiple |
| 55 | dentries (hard links, for example, do this). |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 56 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 57 | To look up an inode requires that the VFS calls the lookup() method of |
| 58 | the parent directory inode. This method is installed by the specific |
| 59 | filesystem implementation that the inode lives in. Once the VFS has |
| 60 | the required dentry (and hence the inode), we can do all those boring |
| 61 | things like open(2) the file, or stat(2) it to peek at the inode |
| 62 | data. The stat(2) operation is fairly simple: once the VFS has the |
| 63 | dentry, it peeks at the inode data and passes some of it back to |
| 64 | userspace. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 65 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 66 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 67 | The File Object |
| 68 | --------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 69 | |
| 70 | Opening a file requires another operation: allocation of a file |
| 71 | structure (this is the kernel-side implementation of file |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 72 | descriptors). The freshly allocated file structure is initialized with |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 73 | a pointer to the dentry and a set of file operation member functions. |
| 74 | These are taken from the inode data. The open() file method is then |
Francis Galiegue | a33f322 | 2010-04-23 00:08:02 +0200 | [diff] [blame] | 75 | called so the specific filesystem implementation can do its work. You |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 76 | can see that this is another switch performed by the VFS. The file |
| 77 | structure is placed into the file descriptor table for the process. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 78 | |
| 79 | Reading, writing and closing files (and other assorted VFS operations) |
| 80 | is done by using the userspace file descriptor to grab the appropriate |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 81 | file structure, and then calling the required file structure method to |
| 82 | do whatever is required. For as long as the file is open, it keeps the |
| 83 | dentry in use, which in turn means that the VFS inode is still in use. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 84 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 85 | |
| 86 | Registering and Mounting a Filesystem |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 87 | ===================================== |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 88 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 89 | To register and unregister a filesystem, use the following API |
| 90 | functions: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 91 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 92 | #include <linux/fs.h> |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 93 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 94 | extern int register_filesystem(struct file_system_type *); |
| 95 | extern int unregister_filesystem(struct file_system_type *); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 96 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 97 | The passed struct file_system_type describes your filesystem. When a |
| 98 | request is made to mount a device onto a directory in your filespace, |
| 99 | the VFS will call the appropriate get_sb() method for the specific |
| 100 | filesystem. The dentry for the mount point will then be updated to |
| 101 | point to the root inode for the new filesystem. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 102 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 103 | You can see all filesystems that are registered to the kernel in the |
| 104 | file /proc/filesystems. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 105 | |
| 106 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 107 | struct file_system_type |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 108 | ----------------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 109 | |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 110 | This describes the filesystem. As of kernel 2.6.22, the following |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 111 | members are defined: |
| 112 | |
| 113 | struct file_system_type { |
| 114 | const char *name; |
| 115 | int fs_flags; |
Jonathan Corbet | 5d8b2eb | 2006-07-10 04:44:07 -0700 | [diff] [blame] | 116 | int (*get_sb) (struct file_system_type *, int, |
| 117 | const char *, void *, struct vfsmount *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 118 | void (*kill_sb) (struct super_block *); |
| 119 | struct module *owner; |
| 120 | struct file_system_type * next; |
| 121 | struct list_head fs_supers; |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 122 | struct lock_class_key s_lock_key; |
| 123 | struct lock_class_key s_umount_key; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 124 | }; |
| 125 | |
| 126 | name: the name of the filesystem type, such as "ext2", "iso9660", |
| 127 | "msdos" and so on |
| 128 | |
| 129 | fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.) |
| 130 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 131 | get_sb: the method to call when a new instance of this |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 132 | filesystem should be mounted |
| 133 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 134 | kill_sb: the method to call when an instance of this filesystem |
| 135 | should be unmounted |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 136 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 137 | owner: for internal VFS use: you should initialize this to THIS_MODULE in |
| 138 | most cases. |
| 139 | |
| 140 | next: for internal VFS use: you should initialize this to NULL |
| 141 | |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 142 | s_lock_key, s_umount_key: lockdep-specific |
| 143 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 144 | The get_sb() method has the following arguments: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 145 | |
Matt LaPlante | d919588 | 2008-07-25 19:45:33 -0700 | [diff] [blame] | 146 | struct file_system_type *fs_type: describes the filesystem, partly initialized |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 147 | by the specific filesystem code |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 148 | |
| 149 | int flags: mount flags |
| 150 | |
| 151 | const char *dev_name: the device name we are mounting. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 152 | |
| 153 | void *data: arbitrary mount options, usually comes as an ASCII |
Miklos Szeredi | f84e3f5 | 2008-02-08 04:21:34 -0800 | [diff] [blame] | 154 | string (see "Mount Options" section) |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 155 | |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 156 | struct vfsmount *mnt: a vfs-internal representation of a mount point |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 157 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 158 | The get_sb() method must determine if the block device specified |
Borislav Petkov | 0746aec | 2007-07-15 23:41:19 -0700 | [diff] [blame] | 159 | in the dev_name and fs_type contains a filesystem of the type the method |
| 160 | supports. If it succeeds in opening the named block device, it initializes a |
| 161 | struct super_block descriptor for the filesystem contained by the block device. |
| 162 | On failure it returns an error. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 163 | |
| 164 | The most interesting member of the superblock structure that the |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 165 | get_sb() method fills in is the "s_op" field. This is a pointer to |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 166 | a "struct super_operations" which describes the next level of the |
| 167 | filesystem implementation. |
| 168 | |
Jim Cromie | e3e1bfe | 2006-01-03 13:35:41 +0100 | [diff] [blame] | 169 | Usually, a filesystem uses one of the generic get_sb() implementations |
| 170 | and provides a fill_super() method instead. The generic methods are: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 171 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 172 | get_sb_bdev: mount a filesystem residing on a block device |
| 173 | |
| 174 | get_sb_nodev: mount a filesystem that is not backed by a device |
| 175 | |
| 176 | get_sb_single: mount a filesystem which shares the instance between |
| 177 | all mounts |
| 178 | |
| 179 | A fill_super() method implementation has the following arguments: |
| 180 | |
| 181 | struct super_block *sb: the superblock structure. The method fill_super() |
| 182 | must initialize this properly. |
| 183 | |
| 184 | void *data: arbitrary mount options, usually comes as an ASCII |
Miklos Szeredi | f84e3f5 | 2008-02-08 04:21:34 -0800 | [diff] [blame] | 185 | string (see "Mount Options" section) |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 186 | |
| 187 | int silent: whether or not to be silent on error |
| 188 | |
| 189 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 190 | The Superblock Object |
| 191 | ===================== |
| 192 | |
| 193 | A superblock object represents a mounted filesystem. |
| 194 | |
| 195 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 196 | struct super_operations |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 197 | ----------------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 198 | |
| 199 | This describes how the VFS can manipulate the superblock of your |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 200 | filesystem. As of kernel 2.6.22, the following members are defined: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 201 | |
| 202 | struct super_operations { |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 203 | struct inode *(*alloc_inode)(struct super_block *sb); |
| 204 | void (*destroy_inode)(struct inode *); |
| 205 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 206 | void (*dirty_inode) (struct inode *); |
| 207 | int (*write_inode) (struct inode *, int); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 208 | void (*drop_inode) (struct inode *); |
| 209 | void (*delete_inode) (struct inode *); |
| 210 | void (*put_super) (struct super_block *); |
| 211 | void (*write_super) (struct super_block *); |
| 212 | int (*sync_fs)(struct super_block *sb, int wait); |
Takashi Sato | c4be0c1 | 2009-01-09 16:40:58 -0800 | [diff] [blame] | 213 | int (*freeze_fs) (struct super_block *); |
| 214 | int (*unfreeze_fs) (struct super_block *); |
David Howells | 726c334 | 2006-06-23 02:02:58 -0700 | [diff] [blame] | 215 | int (*statfs) (struct dentry *, struct kstatfs *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 216 | int (*remount_fs) (struct super_block *, int *, char *); |
| 217 | void (*clear_inode) (struct inode *); |
| 218 | void (*umount_begin) (struct super_block *); |
| 219 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 220 | int (*show_options)(struct seq_file *, struct vfsmount *); |
| 221 | |
| 222 | ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); |
| 223 | ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 224 | }; |
| 225 | |
| 226 | All methods are called without any locks being held, unless otherwise |
| 227 | noted. This means that most methods can block safely. All methods are |
| 228 | only called from a process context (i.e. not from an interrupt handler |
| 229 | or bottom half). |
| 230 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 231 | alloc_inode: this method is called by inode_alloc() to allocate memory |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 232 | for struct inode and initialize it. If this function is not |
| 233 | defined, a simple 'struct inode' is allocated. Normally |
| 234 | alloc_inode will be used to allocate a larger structure which |
| 235 | contains a 'struct inode' embedded within it. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 236 | |
| 237 | destroy_inode: this method is called by destroy_inode() to release |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 238 | resources allocated for struct inode. It is only required if |
| 239 | ->alloc_inode was defined and simply undoes anything done by |
| 240 | ->alloc_inode. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 241 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 242 | dirty_inode: this method is called by the VFS to mark an inode dirty. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 243 | |
| 244 | write_inode: this method is called when the VFS needs to write an |
| 245 | inode to disc. The second parameter indicates whether the write |
| 246 | should be synchronous or not, not all filesystems check this flag. |
| 247 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 248 | drop_inode: called when the last access to the inode is dropped, |
| 249 | with the inode_lock spinlock held. |
| 250 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 251 | This method should be either NULL (normal UNIX filesystem |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 252 | semantics) or "generic_delete_inode" (for filesystems that do not |
| 253 | want to cache inodes - causing "delete_inode" to always be |
| 254 | called regardless of the value of i_nlink) |
| 255 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 256 | The "generic_delete_inode()" behavior is equivalent to the |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 257 | old practice of using "force_delete" in the put_inode() case, |
| 258 | but does not have the races that the "force_delete()" approach |
| 259 | had. |
| 260 | |
| 261 | delete_inode: called when the VFS wants to delete an inode |
| 262 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 263 | put_super: called when the VFS wishes to free the superblock |
| 264 | (i.e. unmount). This is called with the superblock lock held |
| 265 | |
| 266 | write_super: called when the VFS superblock needs to be written to |
| 267 | disc. This method is optional |
| 268 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 269 | sync_fs: called when VFS is writing out all dirty data associated with |
| 270 | a superblock. The second parameter indicates whether the method |
| 271 | should wait until the write out has been completed. Optional. |
| 272 | |
Takashi Sato | c4be0c1 | 2009-01-09 16:40:58 -0800 | [diff] [blame] | 273 | freeze_fs: called when VFS is locking a filesystem and |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 274 | forcing it into a consistent state. This method is currently |
| 275 | used by the Logical Volume Manager (LVM). |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 276 | |
Takashi Sato | c4be0c1 | 2009-01-09 16:40:58 -0800 | [diff] [blame] | 277 | unfreeze_fs: called when VFS is unlocking a filesystem and making it writable |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 278 | again. |
| 279 | |
Adrian McMenamin | 66672fe | 2009-04-20 18:38:28 -0700 | [diff] [blame] | 280 | statfs: called when the VFS needs to get filesystem statistics. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 281 | |
| 282 | remount_fs: called when the filesystem is remounted. This is called |
| 283 | with the kernel lock held |
| 284 | |
| 285 | clear_inode: called then the VFS clears the inode. Optional |
| 286 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 287 | umount_begin: called when the VFS is unmounting a filesystem. |
| 288 | |
Miklos Szeredi | f84e3f5 | 2008-02-08 04:21:34 -0800 | [diff] [blame] | 289 | show_options: called by the VFS to show mount options for |
| 290 | /proc/<pid>/mounts. (see "Mount Options" section) |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 291 | |
| 292 | quota_read: called by the VFS to read from filesystem quota file. |
| 293 | |
| 294 | quota_write: called by the VFS to write to filesystem quota file. |
| 295 | |
David Howells | 12debc4 | 2008-02-07 00:15:52 -0800 | [diff] [blame] | 296 | Whoever sets up the inode is responsible for filling in the "i_op" field. This |
| 297 | is a pointer to a "struct inode_operations" which describes the methods that |
| 298 | can be performed on individual inodes. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 299 | |
| 300 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 301 | The Inode Object |
| 302 | ================ |
| 303 | |
| 304 | An inode object represents an object within the filesystem. |
| 305 | |
| 306 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 307 | struct inode_operations |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 308 | ----------------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 309 | |
| 310 | This describes how the VFS can manipulate an inode in your |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 311 | filesystem. As of kernel 2.6.22, the following members are defined: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 312 | |
| 313 | struct inode_operations { |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 314 | int (*create) (struct inode *,struct dentry *,int, struct nameidata *); |
| 315 | struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 316 | int (*link) (struct dentry *,struct inode *,struct dentry *); |
| 317 | int (*unlink) (struct inode *,struct dentry *); |
| 318 | int (*symlink) (struct inode *,struct dentry *,const char *); |
| 319 | int (*mkdir) (struct inode *,struct dentry *,int); |
| 320 | int (*rmdir) (struct inode *,struct dentry *); |
| 321 | int (*mknod) (struct inode *,struct dentry *,int,dev_t); |
| 322 | int (*rename) (struct inode *, struct dentry *, |
| 323 | struct inode *, struct dentry *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 324 | int (*readlink) (struct dentry *, char __user *,int); |
| 325 | void * (*follow_link) (struct dentry *, struct nameidata *); |
| 326 | void (*put_link) (struct dentry *, struct nameidata *, void *); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 327 | void (*truncate) (struct inode *); |
Nick Piggin | b74c79e | 2011-01-07 17:49:58 +1100 | [diff] [blame] | 328 | int (*permission) (struct inode *, int, unsigned int); |
| 329 | int (*check_acl)(struct inode *, int, unsigned int); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 330 | int (*setattr) (struct dentry *, struct iattr *); |
| 331 | int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *); |
| 332 | int (*setxattr) (struct dentry *, const char *,const void *,size_t,int); |
| 333 | ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t); |
| 334 | ssize_t (*listxattr) (struct dentry *, char *, size_t); |
| 335 | int (*removexattr) (struct dentry *, const char *); |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 336 | void (*truncate_range)(struct inode *, loff_t, loff_t); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 337 | }; |
| 338 | |
| 339 | Again, all methods are called without any locks being held, unless |
| 340 | otherwise noted. |
| 341 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 342 | create: called by the open(2) and creat(2) system calls. Only |
| 343 | required if you want to support regular files. The dentry you |
| 344 | get should not have an inode (i.e. it should be a negative |
| 345 | dentry). Here you will probably call d_instantiate() with the |
| 346 | dentry and the newly created inode |
| 347 | |
| 348 | lookup: called when the VFS needs to look up an inode in a parent |
| 349 | directory. The name to look for is found in the dentry. This |
| 350 | method must call d_add() to insert the found inode into the |
| 351 | dentry. The "i_count" field in the inode structure should be |
| 352 | incremented. If the named inode does not exist a NULL inode |
| 353 | should be inserted into the dentry (this is called a negative |
| 354 | dentry). Returning an error code from this routine must only |
| 355 | be done on a real error, otherwise creating inodes with system |
| 356 | calls like create(2), mknod(2), mkdir(2) and so on will fail. |
| 357 | If you wish to overload the dentry methods then you should |
| 358 | initialise the "d_dop" field in the dentry; this is a pointer |
| 359 | to a struct "dentry_operations". |
| 360 | This method is called with the directory inode semaphore held |
| 361 | |
| 362 | link: called by the link(2) system call. Only required if you want |
| 363 | to support hard links. You will probably need to call |
| 364 | d_instantiate() just as you would in the create() method |
| 365 | |
| 366 | unlink: called by the unlink(2) system call. Only required if you |
| 367 | want to support deleting inodes |
| 368 | |
| 369 | symlink: called by the symlink(2) system call. Only required if you |
| 370 | want to support symlinks. You will probably need to call |
| 371 | d_instantiate() just as you would in the create() method |
| 372 | |
| 373 | mkdir: called by the mkdir(2) system call. Only required if you want |
| 374 | to support creating subdirectories. You will probably need to |
| 375 | call d_instantiate() just as you would in the create() method |
| 376 | |
| 377 | rmdir: called by the rmdir(2) system call. Only required if you want |
| 378 | to support deleting subdirectories |
| 379 | |
| 380 | mknod: called by the mknod(2) system call to create a device (char, |
| 381 | block) inode or a named pipe (FIFO) or socket. Only required |
| 382 | if you want to support creating these types of inodes. You |
| 383 | will probably need to call d_instantiate() just as you would |
| 384 | in the create() method |
| 385 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 386 | rename: called by the rename(2) system call to rename the object to |
| 387 | have the parent and name given by the second inode and dentry. |
| 388 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 389 | readlink: called by the readlink(2) system call. Only required if |
| 390 | you want to support reading symbolic links |
| 391 | |
| 392 | follow_link: called by the VFS to follow a symbolic link to the |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 393 | inode it points to. Only required if you want to support |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 394 | symbolic links. This method returns a void pointer cookie |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 395 | that is passed to put_link(). |
| 396 | |
| 397 | put_link: called by the VFS to release resources allocated by |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 398 | follow_link(). The cookie returned by follow_link() is passed |
Paolo Ornati | 670e9f3 | 2006-10-03 22:57:56 +0200 | [diff] [blame] | 399 | to this method as the last parameter. It is used by |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 400 | filesystems such as NFS where page cache is not stable |
| 401 | (i.e. page that was installed when the symbolic link walk |
| 402 | started might not be in the page cache at the end of the |
| 403 | walk). |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 404 | |
npiggin@suse.de | 7bb46a6 | 2010-05-27 01:05:33 +1000 | [diff] [blame] | 405 | truncate: Deprecated. This will not be called if ->setsize is defined. |
| 406 | Called by the VFS to change the size of a file. The |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 407 | i_size field of the inode is set to the desired size by the |
| 408 | VFS before this method is called. This method is called by |
| 409 | the truncate(2) system call and related functionality. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 410 | |
npiggin@suse.de | 7bb46a6 | 2010-05-27 01:05:33 +1000 | [diff] [blame] | 411 | Note: ->truncate and vmtruncate are deprecated. Do not add new |
| 412 | instances/calls of these. Filesystems should be converted to do their |
| 413 | truncate sequence via ->setattr(). |
| 414 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 415 | permission: called by the VFS to check for access rights on a POSIX-like |
| 416 | filesystem. |
| 417 | |
Nick Piggin | a82416d | 2011-01-14 02:26:53 +0000 | [diff] [blame] | 418 | May be called in rcu-walk mode (flags & IPERM_FLAG_RCU). If in rcu-walk |
| 419 | mode, the filesystem must check the permission without blocking or |
Nick Piggin | b74c79e | 2011-01-07 17:49:58 +1100 | [diff] [blame] | 420 | storing to the inode. |
| 421 | |
| 422 | If a situation is encountered that rcu-walk cannot handle, return |
| 423 | -ECHILD and it will be called again in ref-walk mode. |
| 424 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 425 | setattr: called by the VFS to set attributes for a file. This method |
| 426 | is called by chmod(2) and related system calls. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 427 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 428 | getattr: called by the VFS to get attributes of a file. This method |
| 429 | is called by stat(2) and related system calls. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 430 | |
| 431 | setxattr: called by the VFS to set an extended attribute for a file. |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 432 | Extended attribute is a name:value pair associated with an |
| 433 | inode. This method is called by setxattr(2) system call. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 434 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 435 | getxattr: called by the VFS to retrieve the value of an extended |
| 436 | attribute name. This method is called by getxattr(2) function |
| 437 | call. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 438 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 439 | listxattr: called by the VFS to list all extended attributes for a |
| 440 | given file. This method is called by listxattr(2) system call. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 441 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 442 | removexattr: called by the VFS to remove an extended attribute from |
| 443 | a file. This method is called by removexattr(2) system call. |
| 444 | |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 445 | truncate_range: a method provided by the underlying filesystem to truncate a |
| 446 | range of blocks , i.e. punch a hole somewhere in a file. |
| 447 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 448 | |
| 449 | The Address Space Object |
| 450 | ======================== |
| 451 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 452 | The address space object is used to group and manage pages in the page |
| 453 | cache. It can be used to keep track of the pages in a file (or |
| 454 | anything else) and also track the mapping of sections of the file into |
| 455 | process address spaces. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 456 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 457 | There are a number of distinct yet related services that an |
| 458 | address-space can provide. These include communicating memory |
| 459 | pressure, page lookup by address, and keeping track of pages tagged as |
| 460 | Dirty or Writeback. |
| 461 | |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 462 | The first can be used independently to the others. The VM can try to |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 463 | either write dirty pages in order to clean them, or release clean |
| 464 | pages in order to reuse them. To do this it can call the ->writepage |
| 465 | method on dirty pages, and ->releasepage on clean pages with |
| 466 | PagePrivate set. Clean pages without PagePrivate and with no external |
| 467 | references will be released without notice being given to the |
| 468 | address_space. |
| 469 | |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 470 | To achieve this functionality, pages need to be placed on an LRU with |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 471 | lru_cache_add and mark_page_active needs to be called whenever the |
| 472 | page is used. |
| 473 | |
| 474 | Pages are normally kept in a radix tree index by ->index. This tree |
| 475 | maintains information about the PG_Dirty and PG_Writeback status of |
| 476 | each page, so that pages with either of these flags can be found |
| 477 | quickly. |
| 478 | |
| 479 | The Dirty tag is primarily used by mpage_writepages - the default |
| 480 | ->writepages method. It uses the tag to find dirty pages to call |
| 481 | ->writepage on. If mpage_writepages is not used (i.e. the address |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 482 | provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 483 | almost unused. write_inode_now and sync_inode do use it (through |
| 484 | __sync_single_inode) to check if ->writepages has been successful in |
| 485 | writing out the whole address_space. |
| 486 | |
| 487 | The Writeback tag is used by filemap*wait* and sync_page* functions, |
Christoph Hellwig | 94004ed | 2009-09-30 22:16:33 +0200 | [diff] [blame] | 488 | via filemap_fdatawait_range, to wait for all writeback to |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 489 | complete. While waiting ->sync_page (if defined) will be called on |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 490 | each page that is found to require writeback. |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 491 | |
| 492 | An address_space handler may attach extra information to a page, |
| 493 | typically using the 'private' field in the 'struct page'. If such |
| 494 | information is attached, the PG_Private flag should be set. This will |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 495 | cause various VM routines to make extra calls into the address_space |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 496 | handler to deal with that data. |
| 497 | |
| 498 | An address space acts as an intermediate between storage and |
| 499 | application. Data is read into the address space a whole page at a |
| 500 | time, and provided to the application either by copying of the page, |
| 501 | or by memory-mapping the page. |
| 502 | Data is written into the address space by the application, and then |
| 503 | written-back to storage typically in whole pages, however the |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 504 | address_space has finer control of write sizes. |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 505 | |
| 506 | The read process essentially only requires 'readpage'. The write |
Nick Piggin | 4e02ed4 | 2008-10-29 14:00:55 -0700 | [diff] [blame] | 507 | process is more complicated and uses write_begin/write_end or |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 508 | set_page_dirty to write data into the address_space, and writepage, |
| 509 | sync_page, and writepages to writeback data to storage. |
| 510 | |
| 511 | Adding and removing pages to/from an address_space is protected by the |
| 512 | inode's i_mutex. |
| 513 | |
| 514 | When data is written to a page, the PG_Dirty flag should be set. It |
| 515 | typically remains set until writepage asks for it to be written. This |
| 516 | should clear PG_Dirty and set PG_Writeback. It can be actually |
| 517 | written at any point after PG_Dirty is clear. Once it is known to be |
| 518 | safe, PG_Writeback is cleared. |
| 519 | |
| 520 | Writeback makes use of a writeback_control structure... |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 521 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 522 | struct address_space_operations |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 523 | ------------------------------- |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 524 | |
| 525 | This describes how the VFS can manipulate mapping of a file to page cache in |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 526 | your filesystem. As of kernel 2.6.22, the following members are defined: |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 527 | |
| 528 | struct address_space_operations { |
| 529 | int (*writepage)(struct page *page, struct writeback_control *wbc); |
| 530 | int (*readpage)(struct file *, struct page *); |
| 531 | int (*sync_page)(struct page *); |
| 532 | int (*writepages)(struct address_space *, struct writeback_control *); |
| 533 | int (*set_page_dirty)(struct page *page); |
| 534 | int (*readpages)(struct file *filp, struct address_space *mapping, |
| 535 | struct list_head *pages, unsigned nr_pages); |
Nick Piggin | afddba4 | 2007-10-16 01:25:01 -0700 | [diff] [blame] | 536 | int (*write_begin)(struct file *, struct address_space *mapping, |
| 537 | loff_t pos, unsigned len, unsigned flags, |
| 538 | struct page **pagep, void **fsdata); |
| 539 | int (*write_end)(struct file *, struct address_space *mapping, |
| 540 | loff_t pos, unsigned len, unsigned copied, |
| 541 | struct page *page, void *fsdata); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 542 | sector_t (*bmap)(struct address_space *, sector_t); |
| 543 | int (*invalidatepage) (struct page *, unsigned long); |
| 544 | int (*releasepage) (struct page *, int); |
Linus Torvalds | 6072d13 | 2010-12-01 13:35:19 -0500 | [diff] [blame] | 545 | void (*freepage)(struct page *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 546 | ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov, |
| 547 | loff_t offset, unsigned long nr_segs); |
| 548 | struct page* (*get_xip_page)(struct address_space *, sector_t, |
| 549 | int); |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 550 | /* migrate the contents of a page to the specified target */ |
| 551 | int (*migratepage) (struct page *, struct page *); |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 552 | int (*launder_page) (struct page *); |
Andi Kleen | 2571873 | 2009-09-16 11:50:13 +0200 | [diff] [blame] | 553 | int (*error_remove_page) (struct mapping *mapping, struct page *page); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 554 | }; |
| 555 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 556 | writepage: called by the VM to write a dirty page to backing store. |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 557 | This may happen for data integrity reasons (i.e. 'sync'), or |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 558 | to free up memory (flush). The difference can be seen in |
| 559 | wbc->sync_mode. |
| 560 | The PG_Dirty flag has been cleared and PageLocked is true. |
| 561 | writepage should start writeout, should set PG_Writeback, |
| 562 | and should make sure the page is unlocked, either synchronously |
| 563 | or asynchronously when the write operation completes. |
| 564 | |
| 565 | If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 566 | try too hard if there are problems, and may choose to write out |
| 567 | other pages from the mapping if that is easier (e.g. due to |
| 568 | internal dependencies). If it chooses not to start writeout, it |
| 569 | should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 570 | calling ->writepage on that page. |
| 571 | |
| 572 | See the file "Locking" for more details. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 573 | |
| 574 | readpage: called by the VM to read a page from backing store. |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 575 | The page will be Locked when readpage is called, and should be |
| 576 | unlocked and marked uptodate once the read completes. |
| 577 | If ->readpage discovers that it needs to unlock the page for |
| 578 | some reason, it can do so, and then return AOP_TRUNCATED_PAGE. |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 579 | In this case, the page will be relocated, relocked and if |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 580 | that all succeeds, ->readpage will be called again. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 581 | |
| 582 | sync_page: called by the VM to notify the backing store to perform all |
| 583 | queued I/O operations for a page. I/O operations for other pages |
| 584 | associated with this address_space object may also be performed. |
| 585 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 586 | This function is optional and is called only for pages with |
| 587 | PG_Writeback set while waiting for the writeback to complete. |
| 588 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 589 | writepages: called by the VM to write out pages associated with the |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 590 | address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then |
| 591 | the writeback_control will specify a range of pages that must be |
| 592 | written out. If it is WBC_SYNC_NONE, then a nr_to_write is given |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 593 | and that many pages should be written if possible. |
| 594 | If no ->writepages is given, then mpage_writepages is used |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 595 | instead. This will choose pages from the address space that are |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 596 | tagged as DIRTY and will pass them to ->writepage. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 597 | |
| 598 | set_page_dirty: called by the VM to set a page dirty. |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 599 | This is particularly needed if an address space attaches |
| 600 | private data to a page, and that data needs to be updated when |
| 601 | a page is dirtied. This is called, for example, when a memory |
| 602 | mapped page gets modified. |
| 603 | If defined, it should set the PageDirty flag, and the |
| 604 | PAGECACHE_TAG_DIRTY tag in the radix tree. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 605 | |
| 606 | readpages: called by the VM to read pages associated with the address_space |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 607 | object. This is essentially just a vector version of |
| 608 | readpage. Instead of just one page, several pages are |
| 609 | requested. |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 610 | readpages is only used for read-ahead, so read errors are |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 611 | ignored. If anything goes wrong, feel free to give up. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 612 | |
Nick Piggin | 4e02ed4 | 2008-10-29 14:00:55 -0700 | [diff] [blame] | 613 | write_begin: |
Nick Piggin | afddba4 | 2007-10-16 01:25:01 -0700 | [diff] [blame] | 614 | Called by the generic buffered write code to ask the filesystem to |
| 615 | prepare to write len bytes at the given offset in the file. The |
| 616 | address_space should check that the write will be able to complete, |
| 617 | by allocating space if necessary and doing any other internal |
| 618 | housekeeping. If the write will update parts of any basic-blocks on |
| 619 | storage, then those blocks should be pre-read (if they haven't been |
| 620 | read already) so that the updated blocks can be written out properly. |
| 621 | |
| 622 | The filesystem must return the locked pagecache page for the specified |
| 623 | offset, in *pagep, for the caller to write into. |
| 624 | |
Nick Piggin | 4e02ed4 | 2008-10-29 14:00:55 -0700 | [diff] [blame] | 625 | It must be able to cope with short writes (where the length passed to |
| 626 | write_begin is greater than the number of bytes copied into the page). |
| 627 | |
Nick Piggin | afddba4 | 2007-10-16 01:25:01 -0700 | [diff] [blame] | 628 | flags is a field for AOP_FLAG_xxx flags, described in |
| 629 | include/linux/fs.h. |
| 630 | |
| 631 | A void * may be returned in fsdata, which then gets passed into |
| 632 | write_end. |
| 633 | |
| 634 | Returns 0 on success; < 0 on failure (which is the error code), in |
| 635 | which case write_end is not called. |
| 636 | |
| 637 | write_end: After a successful write_begin, and data copy, write_end must |
| 638 | be called. len is the original len passed to write_begin, and copied |
| 639 | is the amount that was able to be copied (copied == len is always true |
| 640 | if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag). |
| 641 | |
| 642 | The filesystem must take care of unlocking the page and releasing it |
| 643 | refcount, and updating i_size. |
| 644 | |
| 645 | Returns < 0 on failure, otherwise the number of bytes (<= 'copied') |
| 646 | that were able to be copied into pagecache. |
| 647 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 648 | bmap: called by the VFS to map a logical block offset within object to |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 649 | physical block number. This method is used by the FIBMAP |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 650 | ioctl and for working with swap-files. To be able to swap to |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 651 | a file, the file must have a stable mapping to a block |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 652 | device. The swap system does not go through the filesystem |
| 653 | but instead uses bmap to find out where the blocks in the file |
| 654 | are and uses those addresses directly. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 655 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 656 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 657 | invalidatepage: If a page has PagePrivate set, then invalidatepage |
| 658 | will be called when part or all of the page is to be removed |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 659 | from the address space. This generally corresponds to either a |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 660 | truncation or a complete invalidation of the address space |
| 661 | (in the latter case 'offset' will always be 0). |
| 662 | Any private data associated with the page should be updated |
| 663 | to reflect this truncation. If offset is 0, then |
| 664 | the private data should be released, because the page |
| 665 | must be able to be completely discarded. This may be done by |
| 666 | calling the ->releasepage function, but in this case the |
| 667 | release MUST succeed. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 668 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 669 | releasepage: releasepage is called on PagePrivate pages to indicate |
| 670 | that the page should be freed if possible. ->releasepage |
| 671 | should remove any private data from the page and clear the |
Andrew Morton | 4fe65ca | 2010-12-02 14:31:19 -0800 | [diff] [blame] | 672 | PagePrivate flag. If releasepage() fails for some reason, it must |
| 673 | indicate failure with a 0 return value. |
| 674 | releasepage() is used in two distinct though related cases. The |
| 675 | first is when the VM finds a clean page with no active users and |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 676 | wants to make it a free page. If ->releasepage succeeds, the |
| 677 | page will be removed from the address_space and become free. |
| 678 | |
Shaun Zinck | bc5b1d5 | 2007-10-20 02:35:36 +0200 | [diff] [blame] | 679 | The second case is when a request has been made to invalidate |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 680 | some or all pages in an address_space. This can happen |
| 681 | through the fadvice(POSIX_FADV_DONTNEED) system call or by the |
| 682 | filesystem explicitly requesting it as nfs and 9fs do (when |
| 683 | they believe the cache may be out of date with storage) by |
| 684 | calling invalidate_inode_pages2(). |
| 685 | If the filesystem makes such a call, and needs to be certain |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 686 | that all pages are invalidated, then its releasepage will |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 687 | need to ensure this. Possibly it can clear the PageUptodate |
| 688 | bit if it cannot free private data yet. |
| 689 | |
Linus Torvalds | 6072d13 | 2010-12-01 13:35:19 -0500 | [diff] [blame] | 690 | freepage: freepage is called once the page is no longer visible in |
| 691 | the page cache in order to allow the cleanup of any private |
| 692 | data. Since it may be called by the memory reclaimer, it |
| 693 | should not assume that the original address_space mapping still |
| 694 | exists, and it should not block. |
| 695 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 696 | direct_IO: called by the generic read/write routines to perform |
| 697 | direct_IO - that is IO requests which bypass the page cache |
NeilBrown | a9e102b | 2006-03-25 03:08:29 -0800 | [diff] [blame] | 698 | and transfer data directly between the storage and the |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 699 | application's address space. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 700 | |
| 701 | get_xip_page: called by the VM to translate a block number to a page. |
| 702 | The page is valid until the corresponding filesystem is unmounted. |
| 703 | Filesystems that want to use execute-in-place (XIP) need to implement |
| 704 | it. An example implementation can be found in fs/ext2/xip.c. |
| 705 | |
NeilBrown | 341546f | 2006-03-25 03:07:56 -0800 | [diff] [blame] | 706 | migrate_page: This is used to compact the physical memory usage. |
| 707 | If the VM wants to relocate a page (maybe off a memory card |
| 708 | that is signalling imminent failure) it will pass a new page |
| 709 | and an old page to this function. migrate_page should |
| 710 | transfer any private data across and update any references |
| 711 | that it has to the page. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 712 | |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 713 | launder_page: Called before freeing a page - it writes back the dirty page. To |
| 714 | prevent redirtying the page, it is kept locked during the whole |
| 715 | operation. |
| 716 | |
Andi Kleen | 2571873 | 2009-09-16 11:50:13 +0200 | [diff] [blame] | 717 | error_remove_page: normally set to generic_error_remove_page if truncation |
| 718 | is ok for this address space. Used for memory failure handling. |
| 719 | Setting this implies you deal with pages going away under you, |
| 720 | unless you have them locked or reference counts increased. |
| 721 | |
| 722 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 723 | The File Object |
| 724 | =============== |
| 725 | |
| 726 | A file object represents a file opened by a process. |
| 727 | |
| 728 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 729 | struct file_operations |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 730 | ---------------------- |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 731 | |
| 732 | This describes how the VFS can manipulate an open file. As of kernel |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 733 | 2.6.22, the following members are defined: |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 734 | |
| 735 | struct file_operations { |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 736 | struct module *owner; |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 737 | loff_t (*llseek) (struct file *, loff_t, int); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 738 | ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 739 | ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); |
Badari Pulavarty | 027445c | 2006-09-30 23:28:46 -0700 | [diff] [blame] | 740 | ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
| 741 | ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 742 | int (*readdir) (struct file *, void *, filldir_t); |
| 743 | unsigned int (*poll) (struct file *, struct poll_table_struct *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 744 | long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); |
| 745 | long (*compat_ioctl) (struct file *, unsigned int, unsigned long); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 746 | int (*mmap) (struct file *, struct vm_area_struct *); |
| 747 | int (*open) (struct inode *, struct file *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 748 | int (*flush) (struct file *); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 749 | int (*release) (struct inode *, struct file *); |
Christoph Hellwig | 7ea8085 | 2010-05-26 17:53:25 +0200 | [diff] [blame] | 750 | int (*fsync) (struct file *, int datasync); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 751 | int (*aio_fsync) (struct kiocb *, int datasync); |
| 752 | int (*fasync) (int, struct file *, int); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 753 | int (*lock) (struct file *, int, struct file_lock *); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 754 | ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *); |
| 755 | ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *); |
| 756 | ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *); |
| 757 | ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); |
| 758 | unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); |
| 759 | int (*check_flags)(int); |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 760 | int (*flock) (struct file *, int, struct file_lock *); |
Borislav Petkov | 422b14c | 2007-07-15 23:41:43 -0700 | [diff] [blame] | 761 | ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int); |
| 762 | ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 763 | }; |
| 764 | |
| 765 | Again, all methods are called without any locks being held, unless |
| 766 | otherwise noted. |
| 767 | |
| 768 | llseek: called when the VFS needs to move the file position index |
| 769 | |
| 770 | read: called by read(2) and related system calls |
| 771 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 772 | aio_read: called by io_submit(2) and other asynchronous I/O operations |
| 773 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 774 | write: called by write(2) and related system calls |
| 775 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 776 | aio_write: called by io_submit(2) and other asynchronous I/O operations |
| 777 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 778 | readdir: called when the VFS needs to read the directory contents |
| 779 | |
| 780 | poll: called by the VFS when a process wants to check if there is |
| 781 | activity on this file and (optionally) go to sleep until there |
| 782 | is activity. Called by the select(2) and poll(2) system calls |
| 783 | |
Arnd Bergmann | b19dd42 | 2010-07-04 00:15:10 +0200 | [diff] [blame] | 784 | unlocked_ioctl: called by the ioctl(2) system call. |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 785 | |
| 786 | compat_ioctl: called by the ioctl(2) system call when 32 bit system calls |
| 787 | are used on 64 bit kernels. |
| 788 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 789 | mmap: called by the mmap(2) system call |
| 790 | |
| 791 | open: called by the VFS when an inode should be opened. When the VFS |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 792 | opens a file, it creates a new "struct file". It then calls the |
| 793 | open method for the newly allocated file structure. You might |
| 794 | think that the open method really belongs in |
| 795 | "struct inode_operations", and you may be right. I think it's |
| 796 | done the way it is because it makes filesystems simpler to |
| 797 | implement. The open() method is a good place to initialize the |
| 798 | "private_data" member in the file structure if you want to point |
| 799 | to a device structure |
| 800 | |
| 801 | flush: called by the close(2) system call to flush a file |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 802 | |
| 803 | release: called when the last reference to an open file is closed |
| 804 | |
| 805 | fsync: called by the fsync(2) system call |
| 806 | |
| 807 | fasync: called by the fcntl(2) system call when asynchronous |
| 808 | (non-blocking) mode is enabled for a file |
| 809 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 810 | lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW |
| 811 | commands |
| 812 | |
| 813 | readv: called by the readv(2) system call |
| 814 | |
| 815 | writev: called by the writev(2) system call |
| 816 | |
| 817 | sendfile: called by the sendfile(2) system call |
| 818 | |
| 819 | get_unmapped_area: called by the mmap(2) system call |
| 820 | |
| 821 | check_flags: called by the fcntl(2) system call for F_SETFL command |
| 822 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 823 | flock: called by the flock(2) system call |
| 824 | |
Pekka J Enberg | d1195c5 | 2006-04-11 14:21:59 +0200 | [diff] [blame] | 825 | splice_write: called by the VFS to splice data from a pipe to a file. This |
| 826 | method is used by the splice(2) system call |
| 827 | |
| 828 | splice_read: called by the VFS to splice data from file to a pipe. This |
| 829 | method is used by the splice(2) system call |
| 830 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 831 | Note that the file operations are implemented by the specific |
| 832 | filesystem in which the inode resides. When opening a device node |
| 833 | (character or block special) most filesystems will call special |
| 834 | support routines in the VFS which will locate the required device |
| 835 | driver information. These support routines replace the filesystem file |
| 836 | operations with those for the device driver, and then proceed to call |
| 837 | the new open() method for the file. This is how opening a device file |
| 838 | in the filesystem eventually ends up calling the device driver open() |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 839 | method. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 840 | |
| 841 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 842 | Directory Entry Cache (dcache) |
| 843 | ============================== |
| 844 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 845 | |
| 846 | struct dentry_operations |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 847 | ------------------------ |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 848 | |
| 849 | This describes how a filesystem can overload the standard dentry |
| 850 | operations. Dentries and the dcache are the domain of the VFS and the |
| 851 | individual filesystem implementations. Device drivers have no business |
| 852 | here. These methods may be set to NULL, as they are either optional or |
Eric Dumazet | c23fbb6 | 2007-05-08 00:26:18 -0700 | [diff] [blame] | 853 | the VFS uses a default. As of kernel 2.6.22, the following members are |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 854 | defined: |
| 855 | |
| 856 | struct dentry_operations { |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 857 | int (*d_revalidate)(struct dentry *, struct nameidata *); |
Nick Piggin | b1e6a01 | 2011-01-07 17:49:28 +1100 | [diff] [blame] | 858 | int (*d_hash)(const struct dentry *, const struct inode *, |
| 859 | struct qstr *); |
Nick Piggin | 621e155 | 2011-01-07 17:49:27 +1100 | [diff] [blame] | 860 | int (*d_compare)(const struct dentry *, const struct inode *, |
| 861 | const struct dentry *, const struct inode *, |
| 862 | unsigned int, const char *, const struct qstr *); |
Nick Piggin | fe15ce4 | 2011-01-07 17:49:23 +1100 | [diff] [blame] | 863 | int (*d_delete)(const struct dentry *); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 864 | void (*d_release)(struct dentry *); |
| 865 | void (*d_iput)(struct dentry *, struct inode *); |
Eric Dumazet | c23fbb6 | 2007-05-08 00:26:18 -0700 | [diff] [blame] | 866 | char *(*d_dname)(struct dentry *, char *, int); |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 867 | }; |
| 868 | |
| 869 | d_revalidate: called when the VFS needs to revalidate a dentry. This |
| 870 | is called whenever a name look-up finds a dentry in the |
| 871 | dcache. Most filesystems leave this as NULL, because all their |
| 872 | dentries in the dcache are valid |
| 873 | |
Nick Piggin | 34286d6 | 2011-01-07 17:49:57 +1100 | [diff] [blame] | 874 | d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU). |
| 875 | If in rcu-walk mode, the filesystem must revalidate the dentry without |
| 876 | blocking or storing to the dentry, d_parent and d_inode should not be |
| 877 | used without care (because they can go NULL), instead nd->inode should |
| 878 | be used. |
| 879 | |
| 880 | If a situation is encountered that rcu-walk cannot handle, return |
| 881 | -ECHILD and it will be called again in ref-walk mode. |
| 882 | |
Nick Piggin | 621e155 | 2011-01-07 17:49:27 +1100 | [diff] [blame] | 883 | d_hash: called when the VFS adds a dentry to the hash table. The first |
| 884 | dentry passed to d_hash is the parent directory that the name is |
Nick Piggin | b1e6a01 | 2011-01-07 17:49:28 +1100 | [diff] [blame] | 885 | to be hashed into. The inode is the dentry's inode. |
| 886 | |
| 887 | Same locking and synchronisation rules as d_compare regarding |
| 888 | what is safe to dereference etc. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 889 | |
Nick Piggin | 621e155 | 2011-01-07 17:49:27 +1100 | [diff] [blame] | 890 | d_compare: called to compare a dentry name with a given name. The first |
| 891 | dentry is the parent of the dentry to be compared, the second is |
| 892 | the parent's inode, then the dentry and inode (may be NULL) of the |
| 893 | child dentry. len and name string are properties of the dentry to be |
| 894 | compared. qstr is the name to compare it with. |
| 895 | |
| 896 | Must be constant and idempotent, and should not take locks if |
| 897 | possible, and should not or store into the dentry or inodes. |
| 898 | Should not dereference pointers outside the dentry or inodes without |
| 899 | lots of care (eg. d_parent, d_inode, d_name should not be used). |
| 900 | |
| 901 | However, our vfsmount is pinned, and RCU held, so the dentries and |
| 902 | inodes won't disappear, neither will our sb or filesystem module. |
| 903 | ->i_sb and ->d_sb may be used. |
| 904 | |
| 905 | It is a tricky calling convention because it needs to be called under |
| 906 | "rcu-walk", ie. without any locks or references on things. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 907 | |
Nick Piggin | fe15ce4 | 2011-01-07 17:49:23 +1100 | [diff] [blame] | 908 | d_delete: called when the last reference to a dentry is dropped and the |
| 909 | dcache is deciding whether or not to cache it. Return 1 to delete |
| 910 | immediately, or 0 to cache the dentry. Default is NULL which means to |
| 911 | always cache a reachable dentry. d_delete must be constant and |
| 912 | idempotent. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 913 | |
| 914 | d_release: called when a dentry is really deallocated |
| 915 | |
| 916 | d_iput: called when a dentry loses its inode (just prior to its |
| 917 | being deallocated). The default when this is NULL is that the |
| 918 | VFS calls iput(). If you define this method, you must call |
| 919 | iput() yourself |
| 920 | |
Eric Dumazet | c23fbb6 | 2007-05-08 00:26:18 -0700 | [diff] [blame] | 921 | d_dname: called when the pathname of a dentry should be generated. |
Matt LaPlante | d919588 | 2008-07-25 19:45:33 -0700 | [diff] [blame] | 922 | Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay |
Eric Dumazet | c23fbb6 | 2007-05-08 00:26:18 -0700 | [diff] [blame] | 923 | pathname generation. (Instead of doing it when dentry is created, |
Matt LaPlante | d919588 | 2008-07-25 19:45:33 -0700 | [diff] [blame] | 924 | it's done only when the path is needed.). Real filesystems probably |
Eric Dumazet | c23fbb6 | 2007-05-08 00:26:18 -0700 | [diff] [blame] | 925 | dont want to use it, because their dentries are present in global |
| 926 | dcache hash, so their hash should be an invariant. As no lock is |
| 927 | held, d_dname() should not try to modify the dentry itself, unless |
| 928 | appropriate SMP safety is used. CAUTION : d_path() logic is quite |
| 929 | tricky. The correct way to return for example "Hello" is to put it |
| 930 | at the end of the buffer, and returns a pointer to the first char. |
| 931 | dynamic_dname() helper function is provided to take care of this. |
| 932 | |
| 933 | Example : |
| 934 | |
| 935 | static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) |
| 936 | { |
| 937 | return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", |
| 938 | dentry->d_inode->i_ino); |
| 939 | } |
| 940 | |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 941 | Each dentry has a pointer to its parent dentry, as well as a hash list |
| 942 | of child dentries. Child dentries are basically like files in a |
| 943 | directory. |
| 944 | |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 945 | |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 946 | Directory Entry Cache API |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 947 | -------------------------- |
| 948 | |
| 949 | There are a number of functions defined which permit a filesystem to |
| 950 | manipulate dentries: |
| 951 | |
| 952 | dget: open a new handle for an existing dentry (this just increments |
| 953 | the usage count) |
| 954 | |
| 955 | dput: close a handle for a dentry (decrements the usage count). If |
Nick Piggin | fe15ce4 | 2011-01-07 17:49:23 +1100 | [diff] [blame] | 956 | the usage count drops to 0, and the dentry is still in its |
| 957 | parent's hash, the "d_delete" method is called to check whether |
| 958 | it should be cached. If it should not be cached, or if the dentry |
| 959 | is not hashed, it is deleted. Otherwise cached dentries are put |
| 960 | into an LRU list to be reclaimed on memory shortage. |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 961 | |
| 962 | d_drop: this unhashes a dentry from its parents hash list. A |
Pekka J Enberg | 5ea626a | 2005-09-09 13:10:19 -0700 | [diff] [blame] | 963 | subsequent call to dput() will deallocate the dentry if its |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 964 | usage count drops to 0 |
| 965 | |
| 966 | d_delete: delete a dentry. If there are no other open references to |
| 967 | the dentry then the dentry is turned into a negative dentry |
| 968 | (the d_iput() method is called). If there are other |
| 969 | references, then d_drop() is called instead |
| 970 | |
| 971 | d_add: add a dentry to its parents hash list and then calls |
| 972 | d_instantiate() |
| 973 | |
| 974 | d_instantiate: add a dentry to the alias hash list for the inode and |
| 975 | updates the "d_inode" member. The "i_count" member in the |
| 976 | inode structure should be set/incremented. If the inode |
| 977 | pointer is NULL, the dentry is called a "negative |
| 978 | dentry". This function is commonly called when an inode is |
| 979 | created for an existing negative dentry |
| 980 | |
| 981 | d_lookup: look up a dentry given its parent and path name component |
| 982 | It looks up the child of that given name from the dcache |
| 983 | hash table. If it is found, the reference count is incremented |
Zhaolei | be42c4c | 2008-12-01 14:34:58 -0800 | [diff] [blame] | 984 | and the dentry is returned. The caller must use dput() |
Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 985 | to free the dentry when it finishes using it. |
| 986 | |
Pekka Enberg | cbf8f0f | 2005-11-07 01:01:09 -0800 | [diff] [blame] | 987 | For further information on dentry locking, please refer to the document |
| 988 | Documentation/filesystems/dentry-locking.txt. |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 989 | |
Miklos Szeredi | f84e3f5 | 2008-02-08 04:21:34 -0800 | [diff] [blame] | 990 | Mount Options |
| 991 | ============= |
| 992 | |
| 993 | Parsing options |
| 994 | --------------- |
| 995 | |
| 996 | On mount and remount the filesystem is passed a string containing a |
| 997 | comma separated list of mount options. The options can have either of |
| 998 | these forms: |
| 999 | |
| 1000 | option |
| 1001 | option=value |
| 1002 | |
| 1003 | The <linux/parser.h> header defines an API that helps parse these |
| 1004 | options. There are plenty of examples on how to use it in existing |
| 1005 | filesystems. |
| 1006 | |
| 1007 | Showing options |
| 1008 | --------------- |
| 1009 | |
| 1010 | If a filesystem accepts mount options, it must define show_options() |
| 1011 | to show all the currently active options. The rules are: |
| 1012 | |
| 1013 | - options MUST be shown which are not default or their values differ |
| 1014 | from the default |
| 1015 | |
| 1016 | - options MAY be shown which are enabled by default or have their |
| 1017 | default value |
| 1018 | |
| 1019 | Options used only internally between a mount helper and the kernel |
| 1020 | (such as file descriptors), or which only have an effect during the |
| 1021 | mounting (such as ones controlling the creation of a journal) are exempt |
| 1022 | from the above rules. |
| 1023 | |
| 1024 | The underlying reason for the above rules is to make sure, that a |
| 1025 | mount can be accurately replicated (e.g. umounting and mounting again) |
| 1026 | based on the information found in /proc/mounts. |
| 1027 | |
| 1028 | A simple method of saving options at mount/remount time and showing |
| 1029 | them is provided with the save_mount_options() and |
| 1030 | generic_show_options() helper functions. Please note, that using |
| 1031 | these may have drawbacks. For more info see header comments for these |
| 1032 | functions in fs/namespace.c. |
Pekka Enberg | cc7d1f8 | 2005-11-07 01:01:08 -0800 | [diff] [blame] | 1033 | |
| 1034 | Resources |
| 1035 | ========= |
| 1036 | |
| 1037 | (Note some of these resources are not up-to-date with the latest kernel |
| 1038 | version.) |
| 1039 | |
| 1040 | Creating Linux virtual filesystems. 2002 |
| 1041 | <http://lwn.net/Articles/13325/> |
| 1042 | |
| 1043 | The Linux Virtual File-system Layer by Neil Brown. 1999 |
| 1044 | <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html> |
| 1045 | |
| 1046 | A tour of the Linux VFS by Michael K. Johnson. 1996 |
| 1047 | <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html> |
| 1048 | |
| 1049 | A small trail through the Linux kernel by Andries Brouwer. 2001 |
| 1050 | <http://www.win.tue.nl/~aeb/linux/vfs/trail.html> |