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-
- Overview of the Linux Virtual File System
-
- Original author: Richard Gooch <rgooch@atnf.csiro.au>
-
- Last updated on June 24, 2007.
-
- Copyright (C) 1999 Richard Gooch
- Copyright (C) 2005 Pekka Enberg
-
- This file is released under the GPLv2.
-
-
-Introduction
-============
-
-The Virtual File System (also known as the Virtual Filesystem Switch)
-is the software layer in the kernel that provides the filesystem
-interface to userspace programs. It also provides an abstraction
-within the kernel which allows different filesystem implementations to
-coexist.
-
-VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so
-on are called from a process context. Filesystem locking is described
-in the document Documentation/filesystems/Locking.
-
-
-Directory Entry Cache (dcache)
-------------------------------
-
-The VFS implements the open(2), stat(2), chmod(2), and similar system
-calls. The pathname argument that is passed to them is used by the VFS
-to search through the directory entry cache (also known as the dentry
-cache or dcache). This provides a very fast look-up mechanism to
-translate a pathname (filename) into a specific dentry. Dentries live
-in RAM and are never saved to disc: they exist only for performance.
-
-The dentry cache is meant to be a view into your entire filespace. As
-most computers cannot fit all dentries in the RAM at the same time,
-some bits of the cache are missing. In order to resolve your pathname
-into a dentry, the VFS may have to resort to creating dentries along
-the way, and then loading the inode. This is done by looking up the
-inode.
-
-
-The Inode Object
-----------------
-
-An individual dentry usually has a pointer to an inode. Inodes are
-filesystem objects such as regular files, directories, FIFOs and other
-beasts. They live either on the disc (for block device filesystems)
-or in the memory (for pseudo filesystems). Inodes that live on the
-disc are copied into the memory when required and changes to the inode
-are written back to disc. A single inode can be pointed to by multiple
-dentries (hard links, for example, do this).
-
-To look up an inode requires that the VFS calls the lookup() method of
-the parent directory inode. This method is installed by the specific
-filesystem implementation that the inode lives in. Once the VFS has
-the required dentry (and hence the inode), we can do all those boring
-things like open(2) the file, or stat(2) it to peek at the inode
-data. The stat(2) operation is fairly simple: once the VFS has the
-dentry, it peeks at the inode data and passes some of it back to
-userspace.
-
-
-The File Object
----------------
-
-Opening a file requires another operation: allocation of a file
-structure (this is the kernel-side implementation of file
-descriptors). The freshly allocated file structure is initialized with
-a pointer to the dentry and a set of file operation member functions.
-These are taken from the inode data. The open() file method is then
-called so the specific filesystem implementation can do its work. You
-can see that this is another switch performed by the VFS. The file
-structure is placed into the file descriptor table for the process.
-
-Reading, writing and closing files (and other assorted VFS operations)
-is done by using the userspace file descriptor to grab the appropriate
-file structure, and then calling the required file structure method to
-do whatever is required. For as long as the file is open, it keeps the
-dentry in use, which in turn means that the VFS inode is still in use.
-
-
-Registering and Mounting a Filesystem
-=====================================
-
-To register and unregister a filesystem, use the following API
-functions:
-
- #include <linux/fs.h>
-
- extern int register_filesystem(struct file_system_type *);
- extern int unregister_filesystem(struct file_system_type *);
-
-The passed struct file_system_type describes your filesystem. When a
-request is made to mount a filesystem onto a directory in your namespace,
-the VFS will call the appropriate mount() method for the specific
-filesystem. New vfsmount referring to the tree returned by ->mount()
-will be attached to the mountpoint, so that when pathname resolution
-reaches the mountpoint it will jump into the root of that vfsmount.
-
-You can see all filesystems that are registered to the kernel in the
-file /proc/filesystems.
-
-
-struct file_system_type
------------------------
-
-This describes the filesystem. As of kernel 2.6.39, the following
-members are defined:
-
-struct file_system_type {
- const char *name;
- int fs_flags;
- struct dentry *(*mount) (struct file_system_type *, int,
- const char *, void *);
- void (*kill_sb) (struct super_block *);
- struct module *owner;
- struct file_system_type * next;
- struct list_head fs_supers;
- struct lock_class_key s_lock_key;
- struct lock_class_key s_umount_key;
-};
-
- name: the name of the filesystem type, such as "ext2", "iso9660",
- "msdos" and so on
-
- fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)
-
- mount: the method to call when a new instance of this
- filesystem should be mounted
-
- kill_sb: the method to call when an instance of this filesystem
- should be shut down
-
- owner: for internal VFS use: you should initialize this to THIS_MODULE in
- most cases.
-
- next: for internal VFS use: you should initialize this to NULL
-
- s_lock_key, s_umount_key: lockdep-specific
-
-The mount() method has the following arguments:
-
- struct file_system_type *fs_type: describes the filesystem, partly initialized
- by the specific filesystem code
-
- int flags: mount flags
-
- const char *dev_name: the device name we are mounting.
-
- void *data: arbitrary mount options, usually comes as an ASCII
- string (see "Mount Options" section)
-
-The mount() method must return the root dentry of the tree requested by
-caller. An active reference to its superblock must be grabbed and the
-superblock must be locked. On failure it should return ERR_PTR(error).
-
-The arguments match those of mount(2) and their interpretation
-depends on filesystem type. E.g. for block filesystems, dev_name is
-interpreted as block device name, that device is opened and if it
-contains a suitable filesystem image the method creates and initializes
-struct super_block accordingly, returning its root dentry to caller.
-
-->mount() may choose to return a subtree of existing filesystem - it
-doesn't have to create a new one. The main result from the caller's
-point of view is a reference to dentry at the root of (sub)tree to
-be attached; creation of new superblock is a common side effect.
-
-The most interesting member of the superblock structure that the
-mount() method fills in is the "s_op" field. This is a pointer to
-a "struct super_operations" which describes the next level of the
-filesystem implementation.
-
-Usually, a filesystem uses one of the generic mount() implementations
-and provides a fill_super() callback instead. The generic variants are:
-
- mount_bdev: mount a filesystem residing on a block device
-
- mount_nodev: mount a filesystem that is not backed by a device
-
- mount_single: mount a filesystem which shares the instance between
- all mounts
-
-A fill_super() callback implementation has the following arguments:
-
- struct super_block *sb: the superblock structure. The callback
- must initialize this properly.
-
- void *data: arbitrary mount options, usually comes as an ASCII
- string (see "Mount Options" section)
-
- int silent: whether or not to be silent on error
-
-
-The Superblock Object
-=====================
-
-A superblock object represents a mounted filesystem.
-
-
-struct super_operations
------------------------
-
-This describes how the VFS can manipulate the superblock of your
-filesystem. As of kernel 2.6.22, the following members are defined:
-
-struct super_operations {
- struct inode *(*alloc_inode)(struct super_block *sb);
- void (*destroy_inode)(struct inode *);
-
- void (*dirty_inode) (struct inode *, int flags);
- int (*write_inode) (struct inode *, int);
- void (*drop_inode) (struct inode *);
- void (*delete_inode) (struct inode *);
- void (*put_super) (struct super_block *);
- void (*write_super) (struct super_block *);
- int (*sync_fs)(struct super_block *sb, int wait);
- int (*freeze_fs) (struct super_block *);
- int (*unfreeze_fs) (struct super_block *);
- int (*statfs) (struct dentry *, struct kstatfs *);
- int (*remount_fs) (struct super_block *, int *, char *);
- void (*clear_inode) (struct inode *);
- void (*umount_begin) (struct super_block *);
-
- int (*show_options)(struct seq_file *, struct dentry *);
-
- ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
- ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
- int (*nr_cached_objects)(struct super_block *);
- void (*free_cached_objects)(struct super_block *, int);
-};
-
-All methods are called without any locks being held, unless otherwise
-noted. This means that most methods can block safely. All methods are
-only called from a process context (i.e. not from an interrupt handler
-or bottom half).
-
- alloc_inode: this method is called by inode_alloc() to allocate memory
- for struct inode and initialize it. If this function is not
- defined, a simple 'struct inode' is allocated. Normally
- alloc_inode will be used to allocate a larger structure which
- contains a 'struct inode' embedded within it.
-
- destroy_inode: this method is called by destroy_inode() to release
- resources allocated for struct inode. It is only required if
- ->alloc_inode was defined and simply undoes anything done by
- ->alloc_inode.
-
- dirty_inode: this method is called by the VFS to mark an inode dirty.
-
- write_inode: this method is called when the VFS needs to write an
- inode to disc. The second parameter indicates whether the write
- should be synchronous or not, not all filesystems check this flag.
-
- drop_inode: called when the last access to the inode is dropped,
- with the inode->i_lock spinlock held.
-
- This method should be either NULL (normal UNIX filesystem
- semantics) or "generic_delete_inode" (for filesystems that do not
- want to cache inodes - causing "delete_inode" to always be
- called regardless of the value of i_nlink)
-
- The "generic_delete_inode()" behavior is equivalent to the
- old practice of using "force_delete" in the put_inode() case,
- but does not have the races that the "force_delete()" approach
- had.
-
- delete_inode: called when the VFS wants to delete an inode
-
- put_super: called when the VFS wishes to free the superblock
- (i.e. unmount). This is called with the superblock lock held
-
- write_super: called when the VFS superblock needs to be written to
- disc. This method is optional
-
- sync_fs: called when VFS is writing out all dirty data associated with
- a superblock. The second parameter indicates whether the method
- should wait until the write out has been completed. Optional.
-
- freeze_fs: called when VFS is locking a filesystem and
- forcing it into a consistent state. This method is currently
- used by the Logical Volume Manager (LVM).
-
- unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
- again.
-
- statfs: called when the VFS needs to get filesystem statistics.
-
- remount_fs: called when the filesystem is remounted. This is called
- with the kernel lock held
-
- clear_inode: called then the VFS clears the inode. Optional
-
- umount_begin: called when the VFS is unmounting a filesystem.
-
- show_options: called by the VFS to show mount options for
- /proc/<pid>/mounts. (see "Mount Options" section)
-
- quota_read: called by the VFS to read from filesystem quota file.
-
- quota_write: called by the VFS to write to filesystem quota file.
-
- nr_cached_objects: called by the sb cache shrinking function for the
- filesystem to return the number of freeable cached objects it contains.
- Optional.
-
- free_cache_objects: called by the sb cache shrinking function for the
- filesystem to scan the number of objects indicated to try to free them.
- Optional, but any filesystem implementing this method needs to also
- implement ->nr_cached_objects for it to be called correctly.
-
- We can't do anything with any errors that the filesystem might
- encountered, hence the void return type. This will never be called if
- the VM is trying to reclaim under GFP_NOFS conditions, hence this
- method does not need to handle that situation itself.
-
- Implementations must include conditional reschedule calls inside any
- scanning loop that is done. This allows the VFS to determine
- appropriate scan batch sizes without having to worry about whether
- implementations will cause holdoff problems due to large scan batch
- sizes.
-
-Whoever sets up the inode is responsible for filling in the "i_op" field. This
-is a pointer to a "struct inode_operations" which describes the methods that
-can be performed on individual inodes.
-
-
-The Inode Object
-================
-
-An inode object represents an object within the filesystem.
-
-
-struct inode_operations
------------------------
-
-This describes how the VFS can manipulate an inode in your
-filesystem. As of kernel 2.6.22, the following members are defined:
-
-struct inode_operations {
- int (*create) (struct inode *,struct dentry *, umode_t, struct nameidata *);
- struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameidata *);
- int (*link) (struct dentry *,struct inode *,struct dentry *);
- int (*unlink) (struct inode *,struct dentry *);
- int (*symlink) (struct inode *,struct dentry *,const char *);
- int (*mkdir) (struct inode *,struct dentry *,umode_t);
- int (*rmdir) (struct inode *,struct dentry *);
- int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
- int (*rename) (struct inode *, struct dentry *,
- struct inode *, struct dentry *);
- int (*readlink) (struct dentry *, char __user *,int);
- void * (*follow_link) (struct dentry *, struct nameidata *);
- void (*put_link) (struct dentry *, struct nameidata *, void *);
- void (*truncate) (struct inode *);
- int (*permission) (struct inode *, int);
- int (*get_acl)(struct inode *, int);
- int (*setattr) (struct dentry *, struct iattr *);
- int (*getattr) (struct vfsmount *mnt, struct dentry *, struct kstat *);
- int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
- ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
- ssize_t (*listxattr) (struct dentry *, char *, size_t);
- int (*removexattr) (struct dentry *, const char *);
- void (*update_time)(struct inode *, struct timespec *, int);
-};
-
-Again, all methods are called without any locks being held, unless
-otherwise noted.
-
- create: called by the open(2) and creat(2) system calls. Only
- required if you want to support regular files. The dentry you
- get should not have an inode (i.e. it should be a negative
- dentry). Here you will probably call d_instantiate() with the
- dentry and the newly created inode
-
- lookup: called when the VFS needs to look up an inode in a parent
- directory. The name to look for is found in the dentry. This
- method must call d_add() to insert the found inode into the
- dentry. The "i_count" field in the inode structure should be
- incremented. If the named inode does not exist a NULL inode
- should be inserted into the dentry (this is called a negative
- dentry). Returning an error code from this routine must only
- be done on a real error, otherwise creating inodes with system
- calls like create(2), mknod(2), mkdir(2) and so on will fail.
- If you wish to overload the dentry methods then you should
- initialise the "d_dop" field in the dentry; this is a pointer
- to a struct "dentry_operations".
- This method is called with the directory inode semaphore held
-
- link: called by the link(2) system call. Only required if you want
- to support hard links. You will probably need to call
- d_instantiate() just as you would in the create() method
-
- unlink: called by the unlink(2) system call. Only required if you
- want to support deleting inodes
-
- symlink: called by the symlink(2) system call. Only required if you
- want to support symlinks. You will probably need to call
- d_instantiate() just as you would in the create() method
-
- mkdir: called by the mkdir(2) system call. Only required if you want
- to support creating subdirectories. You will probably need to
- call d_instantiate() just as you would in the create() method
-
- rmdir: called by the rmdir(2) system call. Only required if you want
- to support deleting subdirectories
-
- mknod: called by the mknod(2) system call to create a device (char,
- block) inode or a named pipe (FIFO) or socket. Only required
- if you want to support creating these types of inodes. You
- will probably need to call d_instantiate() just as you would
- in the create() method
-
- rename: called by the rename(2) system call to rename the object to
- have the parent and name given by the second inode and dentry.
-
- readlink: called by the readlink(2) system call. Only required if
- you want to support reading symbolic links
-
- follow_link: called by the VFS to follow a symbolic link to the
- inode it points to. Only required if you want to support
- symbolic links. This method returns a void pointer cookie
- that is passed to put_link().
-
- put_link: called by the VFS to release resources allocated by
- follow_link(). The cookie returned by follow_link() is passed
- to this method as the last parameter. It is used by
- filesystems such as NFS where page cache is not stable
- (i.e. page that was installed when the symbolic link walk
- started might not be in the page cache at the end of the
- walk).
-
- truncate: Deprecated. This will not be called if ->setsize is defined.
- Called by the VFS to change the size of a file. The
- i_size field of the inode is set to the desired size by the
- VFS before this method is called. This method is called by
- the truncate(2) system call and related functionality.
-
- Note: ->truncate and vmtruncate are deprecated. Do not add new
- instances/calls of these. Filesystems should be converted to do their
- truncate sequence via ->setattr().
-
- permission: called by the VFS to check for access rights on a POSIX-like
- filesystem.
-
- May be called in rcu-walk mode (mask & MAY_NOT_BLOCK). If in rcu-walk
- mode, the filesystem must check the permission without blocking or
- storing to the inode.
-
- If a situation is encountered that rcu-walk cannot handle, return
- -ECHILD and it will be called again in ref-walk mode.
-
- setattr: called by the VFS to set attributes for a file. This method
- is called by chmod(2) and related system calls.
-
- getattr: called by the VFS to get attributes of a file. This method
- is called by stat(2) and related system calls.
-
- setxattr: called by the VFS to set an extended attribute for a file.
- Extended attribute is a name:value pair associated with an
- inode. This method is called by setxattr(2) system call.
-
- getxattr: called by the VFS to retrieve the value of an extended
- attribute name. This method is called by getxattr(2) function
- call.
-
- listxattr: called by the VFS to list all extended attributes for a
- given file. This method is called by listxattr(2) system call.
-
- removexattr: called by the VFS to remove an extended attribute from
- a file. This method is called by removexattr(2) system call.
-
- update_time: called by the VFS to update a specific time or the i_version of
- an inode. If this is not defined the VFS will update the inode itself
- and call mark_inode_dirty_sync.
-
-The Address Space Object
-========================
-
-The address space object is used to group and manage pages in the page
-cache. It can be used to keep track of the pages in a file (or
-anything else) and also track the mapping of sections of the file into
-process address spaces.
-
-There are a number of distinct yet related services that an
-address-space can provide. These include communicating memory
-pressure, page lookup by address, and keeping track of pages tagged as
-Dirty or Writeback.
-
-The first can be used independently to the others. The VM can try to
-either write dirty pages in order to clean them, or release clean
-pages in order to reuse them. To do this it can call the ->writepage
-method on dirty pages, and ->releasepage on clean pages with
-PagePrivate set. Clean pages without PagePrivate and with no external
-references will be released without notice being given to the
-address_space.
-
-To achieve this functionality, pages need to be placed on an LRU with
-lru_cache_add and mark_page_active needs to be called whenever the
-page is used.
-
-Pages are normally kept in a radix tree index by ->index. This tree
-maintains information about the PG_Dirty and PG_Writeback status of
-each page, so that pages with either of these flags can be found
-quickly.
-
-The Dirty tag is primarily used by mpage_writepages - the default
-->writepages method. It uses the tag to find dirty pages to call
-->writepage on. If mpage_writepages is not used (i.e. the address
-provides its own ->writepages) , the PAGECACHE_TAG_DIRTY tag is
-almost unused. write_inode_now and sync_inode do use it (through
-__sync_single_inode) to check if ->writepages has been successful in
-writing out the whole address_space.
-
-The Writeback tag is used by filemap*wait* and sync_page* functions,
-via filemap_fdatawait_range, to wait for all writeback to
-complete. While waiting ->sync_page (if defined) will be called on
-each page that is found to require writeback.
-
-An address_space handler may attach extra information to a page,
-typically using the 'private' field in the 'struct page'. If such
-information is attached, the PG_Private flag should be set. This will
-cause various VM routines to make extra calls into the address_space
-handler to deal with that data.
-
-An address space acts as an intermediate between storage and
-application. Data is read into the address space a whole page at a
-time, and provided to the application either by copying of the page,
-or by memory-mapping the page.
-Data is written into the address space by the application, and then
-written-back to storage typically in whole pages, however the
-address_space has finer control of write sizes.
-
-The read process essentially only requires 'readpage'. The write
-process is more complicated and uses write_begin/write_end or
-set_page_dirty to write data into the address_space, and writepage,
-sync_page, and writepages to writeback data to storage.
-
-Adding and removing pages to/from an address_space is protected by the
-inode's i_mutex.
-
-When data is written to a page, the PG_Dirty flag should be set. It
-typically remains set until writepage asks for it to be written. This
-should clear PG_Dirty and set PG_Writeback. It can be actually
-written at any point after PG_Dirty is clear. Once it is known to be
-safe, PG_Writeback is cleared.
-
-Writeback makes use of a writeback_control structure...
-
-struct address_space_operations
--------------------------------
-
-This describes how the VFS can manipulate mapping of a file to page cache in
-your filesystem. As of kernel 2.6.22, the following members are defined:
-
-struct address_space_operations {
- int (*writepage)(struct page *page, struct writeback_control *wbc);
- int (*readpage)(struct file *, struct page *);
- int (*sync_page)(struct page *);
- int (*writepages)(struct address_space *, struct writeback_control *);
- int (*set_page_dirty)(struct page *page);
- int (*readpages)(struct file *filp, struct address_space *mapping,
- struct list_head *pages, unsigned nr_pages);
- int (*write_begin)(struct file *, struct address_space *mapping,
- loff_t pos, unsigned len, unsigned flags,
- struct page **pagep, void **fsdata);
- int (*write_end)(struct file *, struct address_space *mapping,
- loff_t pos, unsigned len, unsigned copied,
- struct page *page, void *fsdata);
- sector_t (*bmap)(struct address_space *, sector_t);
- int (*invalidatepage) (struct page *, unsigned long);
- int (*releasepage) (struct page *, int);
- void (*freepage)(struct page *);
- ssize_t (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
- loff_t offset, unsigned long nr_segs);
- struct page* (*get_xip_page)(struct address_space *, sector_t,
- int);
- /* migrate the contents of a page to the specified target */
- int (*migratepage) (struct page *, struct page *);
- int (*launder_page) (struct page *);
- int (*error_remove_page) (struct mapping *mapping, struct page *page);
-};
-
- writepage: called by the VM to write a dirty page to backing store.
- This may happen for data integrity reasons (i.e. 'sync'), or
- to free up memory (flush). The difference can be seen in
- wbc->sync_mode.
- The PG_Dirty flag has been cleared and PageLocked is true.
- writepage should start writeout, should set PG_Writeback,
- and should make sure the page is unlocked, either synchronously
- or asynchronously when the write operation completes.
-
- If wbc->sync_mode is WB_SYNC_NONE, ->writepage doesn't have to
- try too hard if there are problems, and may choose to write out
- other pages from the mapping if that is easier (e.g. due to
- internal dependencies). If it chooses not to start writeout, it
- should return AOP_WRITEPAGE_ACTIVATE so that the VM will not keep
- calling ->writepage on that page.
-
- See the file "Locking" for more details.
-
- readpage: called by the VM to read a page from backing store.
- The page will be Locked when readpage is called, and should be
- unlocked and marked uptodate once the read completes.
- If ->readpage discovers that it needs to unlock the page for
- some reason, it can do so, and then return AOP_TRUNCATED_PAGE.
- In this case, the page will be relocated, relocked and if
- that all succeeds, ->readpage will be called again.
-
- sync_page: called by the VM to notify the backing store to perform all
- queued I/O operations for a page. I/O operations for other pages
- associated with this address_space object may also be performed.
-
- This function is optional and is called only for pages with
- PG_Writeback set while waiting for the writeback to complete.
-
- writepages: called by the VM to write out pages associated with the
- address_space object. If wbc->sync_mode is WBC_SYNC_ALL, then
- the writeback_control will specify a range of pages that must be
- written out. If it is WBC_SYNC_NONE, then a nr_to_write is given
- and that many pages should be written if possible.
- If no ->writepages is given, then mpage_writepages is used
- instead. This will choose pages from the address space that are
- tagged as DIRTY and will pass them to ->writepage.
-
- set_page_dirty: called by the VM to set a page dirty.
- This is particularly needed if an address space attaches
- private data to a page, and that data needs to be updated when
- a page is dirtied. This is called, for example, when a memory
- mapped page gets modified.
- If defined, it should set the PageDirty flag, and the
- PAGECACHE_TAG_DIRTY tag in the radix tree.
-
- readpages: called by the VM to read pages associated with the address_space
- object. This is essentially just a vector version of
- readpage. Instead of just one page, several pages are
- requested.
- readpages is only used for read-ahead, so read errors are
- ignored. If anything goes wrong, feel free to give up.
-
- write_begin:
- Called by the generic buffered write code to ask the filesystem to
- prepare to write len bytes at the given offset in the file. The
- address_space should check that the write will be able to complete,
- by allocating space if necessary and doing any other internal
- housekeeping. If the write will update parts of any basic-blocks on
- storage, then those blocks should be pre-read (if they haven't been
- read already) so that the updated blocks can be written out properly.
-
- The filesystem must return the locked pagecache page for the specified
- offset, in *pagep, for the caller to write into.
-
- It must be able to cope with short writes (where the length passed to
- write_begin is greater than the number of bytes copied into the page).
-
- flags is a field for AOP_FLAG_xxx flags, described in
- include/linux/fs.h.
-
- A void * may be returned in fsdata, which then gets passed into
- write_end.
-
- Returns 0 on success; < 0 on failure (which is the error code), in
- which case write_end is not called.
-
- write_end: After a successful write_begin, and data copy, write_end must
- be called. len is the original len passed to write_begin, and copied
- is the amount that was able to be copied (copied == len is always true
- if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
-
- The filesystem must take care of unlocking the page and releasing it
- refcount, and updating i_size.
-
- Returns < 0 on failure, otherwise the number of bytes (<= 'copied')
- that were able to be copied into pagecache.
-
- bmap: called by the VFS to map a logical block offset within object to
- physical block number. This method is used by the FIBMAP
- ioctl and for working with swap-files. To be able to swap to
- a file, the file must have a stable mapping to a block
- device. The swap system does not go through the filesystem
- but instead uses bmap to find out where the blocks in the file
- are and uses those addresses directly.
-
-
- invalidatepage: If a page has PagePrivate set, then invalidatepage
- will be called when part or all of the page is to be removed
- from the address space. This generally corresponds to either a
- truncation or a complete invalidation of the address space
- (in the latter case 'offset' will always be 0).
- Any private data associated with the page should be updated
- to reflect this truncation. If offset is 0, then
- the private data should be released, because the page
- must be able to be completely discarded. This may be done by
- calling the ->releasepage function, but in this case the
- release MUST succeed.
-
- releasepage: releasepage is called on PagePrivate pages to indicate
- that the page should be freed if possible. ->releasepage
- should remove any private data from the page and clear the
- PagePrivate flag. If releasepage() fails for some reason, it must
- indicate failure with a 0 return value.
- releasepage() is used in two distinct though related cases. The
- first is when the VM finds a clean page with no active users and
- wants to make it a free page. If ->releasepage succeeds, the
- page will be removed from the address_space and become free.
-
- The second case is when a request has been made to invalidate
- some or all pages in an address_space. This can happen
- through the fadvice(POSIX_FADV_DONTNEED) system call or by the
- filesystem explicitly requesting it as nfs and 9fs do (when
- they believe the cache may be out of date with storage) by
- calling invalidate_inode_pages2().
- If the filesystem makes such a call, and needs to be certain
- that all pages are invalidated, then its releasepage will
- need to ensure this. Possibly it can clear the PageUptodate
- bit if it cannot free private data yet.
-
- freepage: freepage is called once the page is no longer visible in
- the page cache in order to allow the cleanup of any private
- data. Since it may be called by the memory reclaimer, it
- should not assume that the original address_space mapping still
- exists, and it should not block.
-
- direct_IO: called by the generic read/write routines to perform
- direct_IO - that is IO requests which bypass the page cache
- and transfer data directly between the storage and the
- application's address space.
-
- get_xip_page: called by the VM to translate a block number to a page.
- The page is valid until the corresponding filesystem is unmounted.
- Filesystems that want to use execute-in-place (XIP) need to implement
- it. An example implementation can be found in fs/ext2/xip.c.
-
- migrate_page: This is used to compact the physical memory usage.
- If the VM wants to relocate a page (maybe off a memory card
- that is signalling imminent failure) it will pass a new page
- and an old page to this function. migrate_page should
- transfer any private data across and update any references
- that it has to the page.
-
- launder_page: Called before freeing a page - it writes back the dirty page. To
- prevent redirtying the page, it is kept locked during the whole
- operation.
-
- error_remove_page: normally set to generic_error_remove_page if truncation
- is ok for this address space. Used for memory failure handling.
- Setting this implies you deal with pages going away under you,
- unless you have them locked or reference counts increased.
-
-
-The File Object
-===============
-
-A file object represents a file opened by a process.
-
-
-struct file_operations
-----------------------
-
-This describes how the VFS can manipulate an open file. As of kernel
-3.5, the following members are defined:
-
-struct file_operations {
- struct module *owner;
- loff_t (*llseek) (struct file *, loff_t, int);
- ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
- ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
- ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
- ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
- int (*readdir) (struct file *, void *, filldir_t);
- unsigned int (*poll) (struct file *, struct poll_table_struct *);
- long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
- long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
- int (*mmap) (struct file *, struct vm_area_struct *);
- int (*open) (struct inode *, struct file *);
- int (*flush) (struct file *);
- int (*release) (struct inode *, struct file *);
- int (*fsync) (struct file *, loff_t, loff_t, int datasync);
- int (*aio_fsync) (struct kiocb *, int datasync);
- int (*fasync) (int, struct file *, int);
- int (*lock) (struct file *, int, struct file_lock *);
- ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
- ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
- ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
- ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
- unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
- int (*check_flags)(int);
- int (*flock) (struct file *, int, struct file_lock *);
- ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
- ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
- int (*setlease)(struct file *, long arg, struct file_lock **);
- long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);
-};
-
-Again, all methods are called without any locks being held, unless
-otherwise noted.
-
- llseek: called when the VFS needs to move the file position index
-
- read: called by read(2) and related system calls
-
- aio_read: called by io_submit(2) and other asynchronous I/O operations
-
- write: called by write(2) and related system calls
-
- aio_write: called by io_submit(2) and other asynchronous I/O operations
-
- readdir: called when the VFS needs to read the directory contents
-
- poll: called by the VFS when a process wants to check if there is
- activity on this file and (optionally) go to sleep until there
- is activity. Called by the select(2) and poll(2) system calls
-
- unlocked_ioctl: called by the ioctl(2) system call.
-
- compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
- are used on 64 bit kernels.
-
- mmap: called by the mmap(2) system call
-
- open: called by the VFS when an inode should be opened. When the VFS
- opens a file, it creates a new "struct file". It then calls the
- open method for the newly allocated file structure. You might
- think that the open method really belongs in
- "struct inode_operations", and you may be right. I think it's
- done the way it is because it makes filesystems simpler to
- implement. The open() method is a good place to initialize the
- "private_data" member in the file structure if you want to point
- to a device structure
-
- flush: called by the close(2) system call to flush a file
-
- release: called when the last reference to an open file is closed
-
- fsync: called by the fsync(2) system call
-
- fasync: called by the fcntl(2) system call when asynchronous
- (non-blocking) mode is enabled for a file
-
- lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW
- commands
-
- readv: called by the readv(2) system call
-
- writev: called by the writev(2) system call
-
- sendfile: called by the sendfile(2) system call
-
- get_unmapped_area: called by the mmap(2) system call
-
- check_flags: called by the fcntl(2) system call for F_SETFL command
-
- flock: called by the flock(2) system call
-
- splice_write: called by the VFS to splice data from a pipe to a file. This
- method is used by the splice(2) system call
-
- splice_read: called by the VFS to splice data from file to a pipe. This
- method is used by the splice(2) system call
-
- setlease: called by the VFS to set or release a file lock lease.
- setlease has the file_lock_lock held and must not sleep.
-
- fallocate: called by the VFS to preallocate blocks or punch a hole.
-
-Note that the file operations are implemented by the specific
-filesystem in which the inode resides. When opening a device node
-(character or block special) most filesystems will call special
-support routines in the VFS which will locate the required device
-driver information. These support routines replace the filesystem file
-operations with those for the device driver, and then proceed to call
-the new open() method for the file. This is how opening a device file
-in the filesystem eventually ends up calling the device driver open()
-method.
-
-
-Directory Entry Cache (dcache)
-==============================
-
-
-struct dentry_operations
-------------------------
-
-This describes how a filesystem can overload the standard dentry
-operations. Dentries and the dcache are the domain of the VFS and the
-individual filesystem implementations. Device drivers have no business
-here. These methods may be set to NULL, as they are either optional or
-the VFS uses a default. As of kernel 2.6.22, the following members are
-defined:
-
-struct dentry_operations {
- int (*d_revalidate)(struct dentry *, struct nameidata *);
- int (*d_hash)(const struct dentry *, const struct inode *,
- struct qstr *);
- int (*d_compare)(const struct dentry *, const struct inode *,
- const struct dentry *, const struct inode *,
- unsigned int, const char *, const struct qstr *);
- int (*d_delete)(const struct dentry *);
- void (*d_release)(struct dentry *);
- void (*d_iput)(struct dentry *, struct inode *);
- char *(*d_dname)(struct dentry *, char *, int);
- struct vfsmount *(*d_automount)(struct path *);
- int (*d_manage)(struct dentry *, bool);
-};
-
- d_revalidate: called when the VFS needs to revalidate a dentry. This
- is called whenever a name look-up finds a dentry in the
- dcache. Most filesystems leave this as NULL, because all their
- dentries in the dcache are valid
-
- d_revalidate may be called in rcu-walk mode (nd->flags & LOOKUP_RCU).
- If in rcu-walk mode, the filesystem must revalidate the dentry without
- blocking or storing to the dentry, d_parent and d_inode should not be
- used without care (because they can go NULL), instead nd->inode should
- be used.
-
- If a situation is encountered that rcu-walk cannot handle, return
- -ECHILD and it will be called again in ref-walk mode.
-
- d_hash: called when the VFS adds a dentry to the hash table. The first
- dentry passed to d_hash is the parent directory that the name is
- to be hashed into. The inode is the dentry's inode.
-
- Same locking and synchronisation rules as d_compare regarding
- what is safe to dereference etc.
-
- d_compare: called to compare a dentry name with a given name. The first
- dentry is the parent of the dentry to be compared, the second is
- the parent's inode, then the dentry and inode (may be NULL) of the
- child dentry. len and name string are properties of the dentry to be
- compared. qstr is the name to compare it with.
-
- Must be constant and idempotent, and should not take locks if
- possible, and should not or store into the dentry or inodes.
- Should not dereference pointers outside the dentry or inodes without
- lots of care (eg. d_parent, d_inode, d_name should not be used).
-
- However, our vfsmount is pinned, and RCU held, so the dentries and
- inodes won't disappear, neither will our sb or filesystem module.
- ->i_sb and ->d_sb may be used.
-
- It is a tricky calling convention because it needs to be called under
- "rcu-walk", ie. without any locks or references on things.
-
- d_delete: called when the last reference to a dentry is dropped and the
- dcache is deciding whether or not to cache it. Return 1 to delete
- immediately, or 0 to cache the dentry. Default is NULL which means to
- always cache a reachable dentry. d_delete must be constant and
- idempotent.
-
- d_release: called when a dentry is really deallocated
-
- d_iput: called when a dentry loses its inode (just prior to its
- being deallocated). The default when this is NULL is that the
- VFS calls iput(). If you define this method, you must call
- iput() yourself
-
- d_dname: called when the pathname of a dentry should be generated.
- Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay
- pathname generation. (Instead of doing it when dentry is created,
- it's done only when the path is needed.). Real filesystems probably
- dont want to use it, because their dentries are present in global
- dcache hash, so their hash should be an invariant. As no lock is
- held, d_dname() should not try to modify the dentry itself, unless
- appropriate SMP safety is used. CAUTION : d_path() logic is quite
- tricky. The correct way to return for example "Hello" is to put it
- at the end of the buffer, and returns a pointer to the first char.
- dynamic_dname() helper function is provided to take care of this.
-
- d_automount: called when an automount dentry is to be traversed (optional).
- This should create a new VFS mount record and return the record to the
- caller. The caller is supplied with a path parameter giving the
- automount directory to describe the automount target and the parent
- VFS mount record to provide inheritable mount parameters. NULL should
- be returned if someone else managed to make the automount first. If
- the vfsmount creation failed, then an error code should be returned.
- If -EISDIR is returned, then the directory will be treated as an
- ordinary directory and returned to pathwalk to continue walking.
-
- If a vfsmount is returned, the caller will attempt to mount it on the
- mountpoint and will remove the vfsmount from its expiration list in
- the case of failure. The vfsmount should be returned with 2 refs on
- it to prevent automatic expiration - the caller will clean up the
- additional ref.
-
- This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
- dentry. This is set by __d_instantiate() if S_AUTOMOUNT is set on the
- inode being added.
-
- d_manage: called to allow the filesystem to manage the transition from a
- dentry (optional). This allows autofs, for example, to hold up clients
- waiting to explore behind a 'mountpoint' whilst letting the daemon go
- past and construct the subtree there. 0 should be returned to let the
- calling process continue. -EISDIR can be returned to tell pathwalk to
- use this directory as an ordinary directory and to ignore anything
- mounted on it and not to check the automount flag. Any other error
- code will abort pathwalk completely.
-
- If the 'rcu_walk' parameter is true, then the caller is doing a
- pathwalk in RCU-walk mode. Sleeping is not permitted in this mode,
- and the caller can be asked to leave it and call again by returning
- -ECHILD.
-
- This function is only used if DCACHE_MANAGE_TRANSIT is set on the
- dentry being transited from.
-
-Example :
-
-static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
-{
- return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
- dentry->d_inode->i_ino);
-}
-
-Each dentry has a pointer to its parent dentry, as well as a hash list
-of child dentries. Child dentries are basically like files in a
-directory.
-
-
-Directory Entry Cache API
---------------------------
-
-There are a number of functions defined which permit a filesystem to
-manipulate dentries:
-
- dget: open a new handle for an existing dentry (this just increments
- the usage count)
-
- dput: close a handle for a dentry (decrements the usage count). If
- the usage count drops to 0, and the dentry is still in its
- parent's hash, the "d_delete" method is called to check whether
- it should be cached. If it should not be cached, or if the dentry
- is not hashed, it is deleted. Otherwise cached dentries are put
- into an LRU list to be reclaimed on memory shortage.
-
- d_drop: this unhashes a dentry from its parents hash list. A
- subsequent call to dput() will deallocate the dentry if its
- usage count drops to 0
-
- d_delete: delete a dentry. If there are no other open references to
- the dentry then the dentry is turned into a negative dentry
- (the d_iput() method is called). If there are other
- references, then d_drop() is called instead
-
- d_add: add a dentry to its parents hash list and then calls
- d_instantiate()
-
- d_instantiate: add a dentry to the alias hash list for the inode and
- updates the "d_inode" member. The "i_count" member in the
- inode structure should be set/incremented. If the inode
- pointer is NULL, the dentry is called a "negative
- dentry". This function is commonly called when an inode is
- created for an existing negative dentry
-
- d_lookup: look up a dentry given its parent and path name component
- It looks up the child of that given name from the dcache
- hash table. If it is found, the reference count is incremented
- and the dentry is returned. The caller must use dput()
- to free the dentry when it finishes using it.
-
-Mount Options
-=============
-
-Parsing options
----------------
-
-On mount and remount the filesystem is passed a string containing a
-comma separated list of mount options. The options can have either of
-these forms:
-
- option
- option=value
-
-The <linux/parser.h> header defines an API that helps parse these
-options. There are plenty of examples on how to use it in existing
-filesystems.
-
-Showing options
----------------
-
-If a filesystem accepts mount options, it must define show_options()
-to show all the currently active options. The rules are:
-
- - options MUST be shown which are not default or their values differ
- from the default
-
- - options MAY be shown which are enabled by default or have their
- default value
-
-Options used only internally between a mount helper and the kernel
-(such as file descriptors), or which only have an effect during the
-mounting (such as ones controlling the creation of a journal) are exempt
-from the above rules.
-
-The underlying reason for the above rules is to make sure, that a
-mount can be accurately replicated (e.g. umounting and mounting again)
-based on the information found in /proc/mounts.
-
-A simple method of saving options at mount/remount time and showing
-them is provided with the save_mount_options() and
-generic_show_options() helper functions. Please note, that using
-these may have drawbacks. For more info see header comments for these
-functions in fs/namespace.c.
-
-Resources
-=========
-
-(Note some of these resources are not up-to-date with the latest kernel
- version.)
-
-Creating Linux virtual filesystems. 2002
- <http://lwn.net/Articles/13325/>
-
-The Linux Virtual File-system Layer by Neil Brown. 1999
- <http://www.cse.unsw.edu.au/~neilb/oss/linux-commentary/vfs.html>
-
-A tour of the Linux VFS by Michael K. Johnson. 1996
- <http://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
-
-A small trail through the Linux kernel by Andries Brouwer. 2001
- <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>