--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================================
+Overview of the Linux Virtual File System
+=========================================
+
+Original author: Richard Gooch <rgooch@atnf.csiro.au>
+
+- Copyright (C) 1999 Richard Gooch
+- Copyright (C) 2005 Pekka Enberg
+
+
+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:
+
+.. code-block:: c
+
+ #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:
+
+.. code-block:: c
+
+ struct file_system_operations {
+ 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:
+
+.. code-block:: c
+
+ 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 *);
+ 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 alloc_inode() 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
+
+``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.
+
+
+struct xattr_handlers
+---------------------
+
+On filesystems that support extended attributes (xattrs), the s_xattr
+superblock field points to a NULL-terminated array of xattr handlers.
+Extended attributes are name:value pairs.
+
+``name``: Indicates that the handler matches attributes with the specified name
+ (such as "system.posix_acl_access"); the prefix field must be NULL.
+
+``prefix``: Indicates that the handler matches all attributes with the specified
+ name prefix (such as "user."); the name field must be NULL.
+
+``list``: Determine if attributes matching this xattr handler should be listed
+ for a particular dentry. Used by some listxattr implementations like
+ generic_listxattr.
+
+``get``: Called by the VFS to get the value of a particular extended attribute.
+ This method is called by the getxattr(2) system call.
+
+``set``: Called by the VFS to set the value of a particular extended attribute.
+ When the new value is NULL, called to remove a particular extended
+ attribute. This method is called by the the setxattr(2) and
+ removexattr(2) system calls.
+
+When none of the xattr handlers of a filesystem match the specified
+attribute name or when a filesystem doesn't support extended attributes,
+the various ``*xattr(2)`` system calls return -EOPNOTSUPP.
+
+
+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:
+
+.. code-block:: c
+
+ struct inode_operations {
+ int (*create) (struct inode *,struct dentry *, umode_t, bool);
+ struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
+ 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 *, unsigned int);
+ int (*readlink) (struct dentry *, char __user *,int);
+ const char *(*get_link) (struct dentry *, struct inode *,
+ struct delayed_call *);
+ int (*permission) (struct inode *, int);
+ int (*get_acl)(struct inode *, int);
+ int (*setattr) (struct dentry *, struct iattr *);
+ int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
+ ssize_t (*listxattr) (struct dentry *, char *, size_t);
+ void (*update_time)(struct inode *, struct timespec *, int);
+ int (*atomic_open)(struct inode *, struct dentry *, struct file *,
+ unsigned open_flag, umode_t create_mode);
+ int (*tmpfile) (struct inode *, struct dentry *, umode_t);
+ };
+
+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.
+
+ The filesystem must return -EINVAL for any unsupported or
+ unknown flags. Currently the following flags are implemented:
+ (1) RENAME_NOREPLACE: this flag indicates that if the target
+ of the rename exists the rename should fail with -EEXIST
+ instead of replacing the target. The VFS already checks for
+ existence, so for local filesystems the RENAME_NOREPLACE
+ implementation is equivalent to plain rename.
+ (2) RENAME_EXCHANGE: exchange source and target. Both must
+ exist; this is checked by the VFS. Unlike plain rename,
+ source and target may be of different type.
+
+``get_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 the symlink body
+ to traverse (and possibly resets the current position with
+ nd_jump_link()). If the body won't go away until the inode
+ is gone, nothing else is needed; if it needs to be otherwise
+ pinned, arrange for its release by having get_link(..., ..., done)
+ do set_delayed_call(done, destructor, argument).
+ In that case destructor(argument) will be called once VFS is
+ done with the body you've returned.
+ May be called in RCU mode; that is indicated by NULL dentry
+ argument. If request can't be handled without leaving RCU mode,
+ have it return ERR_PTR(-ECHILD).
+
+
+ If the filesystem stores the symlink target in ->i_link, the
+ VFS may use it directly without calling ->get_link(); however,
+ ->get_link() must still be provided. ->i_link must not be
+ freed until after an RCU grace period. Writing to ->i_link
+ post-iget() time requires a 'release' memory barrier.
+
+``readlink``: this is now just an override for use by readlink(2) for the
+ cases when ->get_link uses nd_jump_link() or object is not in
+ fact a symlink. Normally filesystems should only implement
+ ->get_link for symlinks and readlink(2) will automatically use
+ that.
+
+``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.
+
+``listxattr``: called by the VFS to list all extended attributes for a
+ given file. This method is called by the listxattr(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.
+
+``atomic_open``: called on the last component of an open. Using this optional
+ method the filesystem can look up, possibly create and open the file in
+ one atomic operation. If it wants to leave actual opening to the
+ caller (e.g. if the file turned out to be a symlink, device, or just
+ something filesystem won't do atomic open for), it may signal this by
+ returning finish_no_open(file, dentry). This method is only called if
+ the last component is negative or needs lookup. Cached positive dentries
+ are still handled by f_op->open(). If the file was created,
+ FMODE_CREATED flag should be set in file->f_mode. In case of O_EXCL
+ the method must only succeed if the file didn't exist and hence FMODE_CREATED
+ shall always be set on success.
+
+``tmpfile``: called in the end of O_TMPFILE open(). Optional, equivalent to
+ atomically creating, opening and unlinking a file in given directory.
+
+
+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.
+
+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 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 to direct the
+operations. This gives the the writepage and writepages operations some
+information about the nature of and reason for the writeback request,
+and the constraints under which it is being done. It is also used to
+return information back to the caller about the result of a writepage or
+writepages request.
+
+
+Handling errors during writeback
+--------------------------------
+
+Most applications that do buffered I/O will periodically call a file
+synchronization call (fsync, fdatasync, msync or sync_file_range) to
+ensure that data written has made it to the backing store. When there
+is an error during writeback, they expect that error to be reported when
+a file sync request is made. After an error has been reported on one
+request, subsequent requests on the same file descriptor should return
+0, unless further writeback errors have occurred since the previous file
+syncronization.
+
+Ideally, the kernel would report errors only on file descriptions on
+which writes were done that subsequently failed to be written back. The
+generic pagecache infrastructure does not track the file descriptions
+that have dirtied each individual page however, so determining which
+file descriptors should get back an error is not possible.
+
+Instead, the generic writeback error tracking infrastructure in the
+kernel settles for reporting errors to fsync on all file descriptions
+that were open at the time that the error occurred. In a situation with
+multiple writers, all of them will get back an error on a subsequent
+fsync, even if all of the writes done through that particular file
+descriptor succeeded (or even if there were no writes on that file
+descriptor at all).
+
+Filesystems that wish to use this infrastructure should call
+mapping_set_error to record the error in the address_space when it
+occurs. Then, after writing back data from the pagecache in their
+file->fsync operation, they should call file_check_and_advance_wb_err to
+ensure that the struct file's error cursor has advanced to the correct
+point in the stream of errors emitted by the backing device(s).
+
+
+struct address_space_operations
+-------------------------------
+
+This describes how the VFS can manipulate mapping of a file to page
+cache in your filesystem. The following members are defined:
+
+.. code-block:: c
+
+ struct address_space_operations {
+ int (*writepage)(struct page *page, struct writeback_control *wbc);
+ int (*readpage)(struct file *, 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);
+ void (*invalidatepage) (struct page *, unsigned int, unsigned int);
+ int (*releasepage) (struct page *, int);
+ void (*freepage)(struct page *);
+ ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
+ /* isolate a page for migration */
+ bool (*isolate_page) (struct page *, isolate_mode_t);
+ /* migrate the contents of a page to the specified target */
+ int (*migratepage) (struct page *, struct page *);
+ /* put migration-failed page back to right list */
+ void (*putback_page) (struct page *);
+ int (*launder_page) (struct page *);
+
+ int (*is_partially_uptodate) (struct page *, unsigned long,
+ unsigned long);
+ void (*is_dirty_writeback) (struct page *, bool *, bool *);
+ int (*error_remove_page) (struct mapping *mapping, struct page *page);
+ int (*swap_activate)(struct file *);
+ int (*swap_deactivate)(struct file *);
+ };
+
+``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.
+
+``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.
+
+ 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, punch hole or a complete invalidation of the address
+ space (in the latter case 'offset' will always be 0 and 'length'
+ will be PAGE_SIZE). Any private data associated with the page
+ should be updated to reflect this truncation. If offset is 0 and
+ length is PAGE_SIZE, 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 fadvise(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.
+
+``isolate_page``: Called by the VM when isolating a movable non-lru page.
+ If page is successfully isolated, VM marks the page as PG_isolated
+ via __SetPageIsolated.
+
+``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.
+
+``putback_page``: Called by the VM when isolated page's migration fails.
+
+``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.
+
+``is_partially_uptodate``: Called by the VM when reading a file through the
+ pagecache when the underlying blocksize != pagesize. If the required
+ block is up to date then the read can complete without needing the IO
+ to bring the whole page up to date.
+
+``is_dirty_writeback``: Called by the VM when attempting to reclaim a page.
+ The VM uses dirty and writeback information to determine if it needs
+ to stall to allow flushers a chance to complete some IO. Ordinarily
+ it can use PageDirty and PageWriteback but some filesystems have
+ more complex state (unstable pages in NFS prevent reclaim) or
+ do not set those flags due to locking problems. This callback
+ allows a filesystem to indicate to the VM if a page should be
+ treated as dirty or writeback for the purposes of stalling.
+
+``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.
+
+``swap_activate``: Called when swapon is used on a file to allocate
+ space if necessary and pin the block lookup information in
+ memory. A return value of zero indicates success,
+ in which case this file can be used to back swapspace.
+
+``swap_deactivate``: Called during swapoff on files where swap_activate
+ was successful.
+
+
+The File Object
+===============
+
+A file object represents a file opened by a process. This is also known
+as an "open file description" in POSIX parlance.
+
+
+struct file_operations
+----------------------
+
+This describes how the VFS can manipulate an open file. As of kernel
+4.18, the following members are defined:
+
+.. code-block:: c
+
+ 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 (*read_iter) (struct kiocb *, struct iov_iter *);
+ ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
+ int (*iopoll)(struct kiocb *kiocb, bool spin);
+ int (*iterate) (struct file *, struct dir_context *);
+ int (*iterate_shared) (struct file *, struct dir_context *);
+ __poll_t (*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 *, fl_owner_t id);
+ int (*release) (struct inode *, struct file *);
+ int (*fsync) (struct file *, loff_t, loff_t, int datasync);
+ int (*fasync) (int, struct file *, int);
+ int (*lock) (struct file *, int, struct file_lock *);
+ 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 *, loff_t *, size_t, unsigned int);
+ ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
+ int (*setlease)(struct file *, long, struct file_lock **, void **);
+ long (*fallocate)(struct file *file, int mode, loff_t offset,
+ loff_t len);
+ void (*show_fdinfo)(struct seq_file *m, struct file *f);
+ #ifndef CONFIG_MMU
+ unsigned (*mmap_capabilities)(struct file *);
+ #endif
+ ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
+ loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
+ struct file *file_out, loff_t pos_out,
+ loff_t len, unsigned int remap_flags);
+ int (*fadvise)(struct file *, loff_t, loff_t, int);
+ };
+
+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
+
+``read_iter``: possibly asynchronous read with iov_iter as destination
+
+``write``: called by write(2) and related system calls
+
+``write_iter``: possibly asynchronous write with iov_iter as source
+
+``iopoll``: called when aio wants to poll for completions on HIPRI iocbs
+
+``iterate``: called when the VFS needs to read the directory contents
+
+``iterate_shared``: called when the VFS needs to read the directory contents
+ when filesystem supports concurrent dir iterators
+
+``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. Also see the section above
+ entitled "Handling errors during writeback".
+
+``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
+
+``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
+ implementations should call generic_setlease to record or remove
+ the lease in the inode after setting it.
+
+``fallocate``: called by the VFS to preallocate blocks or punch a hole.
+
+``copy_file_range``: called by the copy_file_range(2) system call.
+
+``remap_file_range``: called by the ioctl(2) system call for FICLONERANGE and
+ FICLONE and FIDEDUPERANGE commands to remap file ranges. An
+ implementation should remap len bytes at pos_in of the source file into
+ the dest file at pos_out. Implementations must handle callers passing
+ in len == 0; this means "remap to the end of the source file". The
+ return value should the number of bytes remapped, or the usual
+ negative error code if errors occurred before any bytes were remapped.
+ The remap_flags parameter accepts REMAP_FILE_* flags. If
+ REMAP_FILE_DEDUP is set then the implementation must only remap if the
+ requested file ranges have identical contents. If REMAP_CAN_SHORTEN is
+ set, the caller is ok with the implementation shortening the request
+ length to satisfy alignment or EOF requirements (or any other reason).
+
+``fadvise``: possibly called by the fadvise64() system call.
+
+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:
+
+.. code-block:: c
+
+ struct dentry_operations {
+ int (*d_revalidate)(struct dentry *, unsigned int);
+ int (*d_weak_revalidate)(struct dentry *, unsigned int);
+ int (*d_hash)(const struct dentry *, struct qstr *);
+ int (*d_compare)(const struct dentry *,
+ unsigned int, const char *, const struct qstr *);
+ int (*d_delete)(const struct dentry *);
+ int (*d_init)(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)(const struct path *, bool);
+ struct dentry *(*d_real)(struct dentry *, const struct inode *);
+ };
+
+``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 local filesystems leave this as NULL, because all their
+ dentries in the dcache are valid. Network filesystems are different
+ since things can change on the server without the client necessarily
+ being aware of it.
+
+ This function should return a positive value if the dentry is still
+ valid, and zero or a negative error code if it isn't.
+
+ d_revalidate may be called in rcu-walk mode (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 change and, in d_inode case, even
+ become NULL under us).
+
+ If a situation is encountered that rcu-walk cannot handle, return
+ -ECHILD and it will be called again in ref-walk mode.
+
+``_weak_revalidate``: called when the VFS needs to revalidate a "jumped" dentry.
+ This is called when a path-walk ends at dentry that was not acquired by
+ doing a lookup in the parent directory. This includes "/", "." and "..",
+ as well as procfs-style symlinks and mountpoint traversal.
+
+ In this case, we are less concerned with whether the dentry is still
+ fully correct, but rather that the inode is still valid. As with
+ d_revalidate, most local filesystems will set this to NULL since their
+ dcache entries are always valid.
+
+ This function has the same return code semantics as d_revalidate.
+
+ d_weak_revalidate is only called after leaving rcu-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.
+
+ 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 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.
+ Should not dereference pointers outside the dentry 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.
+ ->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_init``: called when a dentry is allocated
+
+``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.
+
+ Example :
+
+.. code-block:: c
+
+ static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
+ {
+ return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
+ dentry->d_inode->i_ino);
+ }
+
+``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' while 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. -EISDIR may also be returned to tell pathwalk to
+ ignore d_automount or any mounts.
+
+ This function is only used if DCACHE_MANAGE_TRANSIT is set on the
+ dentry being transited from.
+
+``d_real``: overlay/union type filesystems implement this method to return one of
+ the underlying dentries hidden by the overlay. It is used in two
+ different modes:
+
+ Called from file_dentry() it returns the real dentry matching the inode
+ argument. The real dentry may be from a lower layer already copied up,
+ but still referenced from the file. This mode is selected with a
+ non-NULL inode argument.
+
+ With NULL inode the topmost real underlying dentry is returned.
+
+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.
+
+
+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>
+++ /dev/null
-.. SPDX-License-Identifier: GPL-2.0
-
-=========================================
-Overview of the Linux Virtual File System
-=========================================
-
-Original author: Richard Gooch <rgooch@atnf.csiro.au>
-
-- Copyright (C) 1999 Richard Gooch
-- Copyright (C) 2005 Pekka Enberg
-
-
-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 *);
- 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 alloc_inode() 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
-
- 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.
-
-
-struct xattr_handlers
----------------------
-
-On filesystems that support extended attributes (xattrs), the s_xattr
-superblock field points to a NULL-terminated array of xattr handlers.
-Extended attributes are name:value pairs.
-
- name: Indicates that the handler matches attributes with the specified name
- (such as "system.posix_acl_access"); the prefix field must be NULL.
-
- prefix: Indicates that the handler matches all attributes with the specified
- name prefix (such as "user."); the name field must be NULL.
-
- list: Determine if attributes matching this xattr handler should be listed
- for a particular dentry. Used by some listxattr implementations like
- generic_listxattr.
-
- get: Called by the VFS to get the value of a particular extended attribute.
- This method is called by the getxattr(2) system call.
-
- set: Called by the VFS to set the value of a particular extended attribute.
- When the new value is NULL, called to remove a particular extended
- attribute. This method is called by the the setxattr(2) and
- removexattr(2) system calls.
-
-When none of the xattr handlers of a filesystem match the specified
-attribute name or when a filesystem doesn't support extended attributes,
-the various *xattr(2) system calls return -EOPNOTSUPP.
-
-
-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, bool);
- struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
- 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 *, unsigned int);
- int (*readlink) (struct dentry *, char __user *,int);
- const char *(*get_link) (struct dentry *, struct inode *,
- struct delayed_call *);
- int (*permission) (struct inode *, int);
- int (*get_acl)(struct inode *, int);
- int (*setattr) (struct dentry *, struct iattr *);
- int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
- ssize_t (*listxattr) (struct dentry *, char *, size_t);
- void (*update_time)(struct inode *, struct timespec *, int);
- int (*atomic_open)(struct inode *, struct dentry *, struct file *,
- unsigned open_flag, umode_t create_mode);
- int (*tmpfile) (struct inode *, struct dentry *, umode_t);
-};
-
-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.
-
- The filesystem must return -EINVAL for any unsupported or
- unknown flags. Currently the following flags are implemented:
- (1) RENAME_NOREPLACE: this flag indicates that if the target
- of the rename exists the rename should fail with -EEXIST
- instead of replacing the target. The VFS already checks for
- existence, so for local filesystems the RENAME_NOREPLACE
- implementation is equivalent to plain rename.
- (2) RENAME_EXCHANGE: exchange source and target. Both must
- exist; this is checked by the VFS. Unlike plain rename,
- source and target may be of different type.
-
- get_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 the symlink body
- to traverse (and possibly resets the current position with
- nd_jump_link()). If the body won't go away until the inode
- is gone, nothing else is needed; if it needs to be otherwise
- pinned, arrange for its release by having get_link(..., ..., done)
- do set_delayed_call(done, destructor, argument).
- In that case destructor(argument) will be called once VFS is
- done with the body you've returned.
- May be called in RCU mode; that is indicated by NULL dentry
- argument. If request can't be handled without leaving RCU mode,
- have it return ERR_PTR(-ECHILD).
-
- If the filesystem stores the symlink target in ->i_link, the
- VFS may use it directly without calling ->get_link(); however,
- ->get_link() must still be provided. ->i_link must not be
- freed until after an RCU grace period. Writing to ->i_link
- post-iget() time requires a 'release' memory barrier.
-
- readlink: this is now just an override for use by readlink(2) for the
- cases when ->get_link uses nd_jump_link() or object is not in
- fact a symlink. Normally filesystems should only implement
- ->get_link for symlinks and readlink(2) will automatically use
- that.
-
- 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.
-
- listxattr: called by the VFS to list all extended attributes for a
- given file. This method is called by the listxattr(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.
-
- atomic_open: called on the last component of an open. Using this optional
- method the filesystem can look up, possibly create and open the file in
- one atomic operation. If it wants to leave actual opening to the
- caller (e.g. if the file turned out to be a symlink, device, or just
- something filesystem won't do atomic open for), it may signal this by
- returning finish_no_open(file, dentry). This method is only called if
- the last component is negative or needs lookup. Cached positive dentries
- are still handled by f_op->open(). If the file was created,
- FMODE_CREATED flag should be set in file->f_mode. In case of O_EXCL
- the method must only succeed if the file didn't exist and hence FMODE_CREATED
- shall always be set on success.
-
- tmpfile: called in the end of O_TMPFILE open(). Optional, equivalent to
- atomically creating, opening and unlinking a file in given directory.
-
-
-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.
-
-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 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 to direct the
-operations. This gives the the writepage and writepages operations some
-information about the nature of and reason for the writeback request,
-and the constraints under which it is being done. It is also used to
-return information back to the caller about the result of a writepage or
-writepages request.
-
-
-Handling errors during writeback
---------------------------------
-
-Most applications that do buffered I/O will periodically call a file
-synchronization call (fsync, fdatasync, msync or sync_file_range) to
-ensure that data written has made it to the backing store. When there
-is an error during writeback, they expect that error to be reported when
-a file sync request is made. After an error has been reported on one
-request, subsequent requests on the same file descriptor should return
-0, unless further writeback errors have occurred since the previous file
-syncronization.
-
-Ideally, the kernel would report errors only on file descriptions on
-which writes were done that subsequently failed to be written back. The
-generic pagecache infrastructure does not track the file descriptions
-that have dirtied each individual page however, so determining which
-file descriptors should get back an error is not possible.
-
-Instead, the generic writeback error tracking infrastructure in the
-kernel settles for reporting errors to fsync on all file descriptions
-that were open at the time that the error occurred. In a situation with
-multiple writers, all of them will get back an error on a subsequent
-fsync, even if all of the writes done through that particular file
-descriptor succeeded (or even if there were no writes on that file
-descriptor at all).
-
-Filesystems that wish to use this infrastructure should call
-mapping_set_error to record the error in the address_space when it
-occurs. Then, after writing back data from the pagecache in their
-file->fsync operation, they should call file_check_and_advance_wb_err to
-ensure that the struct file's error cursor has advanced to the correct
-point in the stream of errors emitted by the backing device(s).
-
-
-struct address_space_operations
--------------------------------
-
-This describes how the VFS can manipulate mapping of a file to page
-cache in your filesystem. 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 (*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);
- void (*invalidatepage) (struct page *, unsigned int, unsigned int);
- int (*releasepage) (struct page *, int);
- void (*freepage)(struct page *);
- ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
- /* isolate a page for migration */
- bool (*isolate_page) (struct page *, isolate_mode_t);
- /* migrate the contents of a page to the specified target */
- int (*migratepage) (struct page *, struct page *);
- /* put migration-failed page back to right list */
- void (*putback_page) (struct page *);
- int (*launder_page) (struct page *);
-
- int (*is_partially_uptodate) (struct page *, unsigned long,
- unsigned long);
- void (*is_dirty_writeback) (struct page *, bool *, bool *);
- int (*error_remove_page) (struct mapping *mapping, struct page *page);
- int (*swap_activate)(struct file *);
- int (*swap_deactivate)(struct file *);
-};
-
- 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.
-
- 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.
-
- 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, punch hole or a complete invalidation of the address
- space (in the latter case 'offset' will always be 0 and 'length'
- will be PAGE_SIZE). Any private data associated with the page
- should be updated to reflect this truncation. If offset is 0 and
- length is PAGE_SIZE, 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 fadvise(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.
-
- isolate_page: Called by the VM when isolating a movable non-lru page.
- If page is successfully isolated, VM marks the page as PG_isolated
- via __SetPageIsolated.
-
- 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.
-
- putback_page: Called by the VM when isolated page's migration fails.
-
- 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.
-
- is_partially_uptodate: Called by the VM when reading a file through the
- pagecache when the underlying blocksize != pagesize. If the required
- block is up to date then the read can complete without needing the IO
- to bring the whole page up to date.
-
- is_dirty_writeback: Called by the VM when attempting to reclaim a page.
- The VM uses dirty and writeback information to determine if it needs
- to stall to allow flushers a chance to complete some IO. Ordinarily
- it can use PageDirty and PageWriteback but some filesystems have
- more complex state (unstable pages in NFS prevent reclaim) or
- do not set those flags due to locking problems. This callback
- allows a filesystem to indicate to the VM if a page should be
- treated as dirty or writeback for the purposes of stalling.
-
- 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.
-
- swap_activate: Called when swapon is used on a file to allocate
- space if necessary and pin the block lookup information in
- memory. A return value of zero indicates success,
- in which case this file can be used to back swapspace.
-
- swap_deactivate: Called during swapoff on files where swap_activate
- was successful.
-
-
-The File Object
-===============
-
-A file object represents a file opened by a process. This is also known
-as an "open file description" in POSIX parlance.
-
-
-struct file_operations
-----------------------
-
-This describes how the VFS can manipulate an open file. As of kernel
-4.18, 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 (*read_iter) (struct kiocb *, struct iov_iter *);
- ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
- int (*iopoll)(struct kiocb *kiocb, bool spin);
- int (*iterate) (struct file *, struct dir_context *);
- int (*iterate_shared) (struct file *, struct dir_context *);
- __poll_t (*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 *, fl_owner_t id);
- int (*release) (struct inode *, struct file *);
- int (*fsync) (struct file *, loff_t, loff_t, int datasync);
- int (*fasync) (int, struct file *, int);
- int (*lock) (struct file *, int, struct file_lock *);
- 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 *, loff_t *, size_t, unsigned int);
- ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
- int (*setlease)(struct file *, long, struct file_lock **, void **);
- long (*fallocate)(struct file *file, int mode, loff_t offset,
- loff_t len);
- void (*show_fdinfo)(struct seq_file *m, struct file *f);
-#ifndef CONFIG_MMU
- unsigned (*mmap_capabilities)(struct file *);
-#endif
- ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
- loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
- struct file *file_out, loff_t pos_out,
- loff_t len, unsigned int remap_flags);
- int (*fadvise)(struct file *, loff_t, loff_t, int);
-};
-
-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
-
- read_iter: possibly asynchronous read with iov_iter as destination
-
- write: called by write(2) and related system calls
-
- write_iter: possibly asynchronous write with iov_iter as source
-
- iopoll: called when aio wants to poll for completions on HIPRI iocbs
-
- iterate: called when the VFS needs to read the directory contents
-
- iterate_shared: called when the VFS needs to read the directory contents
- when filesystem supports concurrent dir iterators
-
- 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. Also see the section above
- entitled "Handling errors during writeback".
-
- 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
-
- 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
- implementations should call generic_setlease to record or remove
- the lease in the inode after setting it.
-
- fallocate: called by the VFS to preallocate blocks or punch a hole.
-
- copy_file_range: called by the copy_file_range(2) system call.
-
- remap_file_range: called by the ioctl(2) system call for FICLONERANGE and
- FICLONE and FIDEDUPERANGE commands to remap file ranges. An
- implementation should remap len bytes at pos_in of the source file into
- the dest file at pos_out. Implementations must handle callers passing
- in len == 0; this means "remap to the end of the source file". The
- return value should the number of bytes remapped, or the usual
- negative error code if errors occurred before any bytes were remapped.
- The remap_flags parameter accepts REMAP_FILE_* flags. If
- REMAP_FILE_DEDUP is set then the implementation must only remap if the
- requested file ranges have identical contents. If REMAP_CAN_SHORTEN is
- set, the caller is ok with the implementation shortening the request
- length to satisfy alignment or EOF requirements (or any other reason).
-
- fadvise: possibly called by the fadvise64() system call.
-
-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 *, unsigned int);
- int (*d_weak_revalidate)(struct dentry *, unsigned int);
- int (*d_hash)(const struct dentry *, struct qstr *);
- int (*d_compare)(const struct dentry *,
- unsigned int, const char *, const struct qstr *);
- int (*d_delete)(const struct dentry *);
- int (*d_init)(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)(const struct path *, bool);
- struct dentry *(*d_real)(struct dentry *, const struct inode *);
-};
-
- 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 local filesystems leave this as NULL, because all their
- dentries in the dcache are valid. Network filesystems are different
- since things can change on the server without the client necessarily
- being aware of it.
-
- This function should return a positive value if the dentry is still
- valid, and zero or a negative error code if it isn't.
-
- d_revalidate may be called in rcu-walk mode (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 change and, in d_inode case, even
- become NULL under us).
-
- If a situation is encountered that rcu-walk cannot handle, return
- -ECHILD and it will be called again in ref-walk mode.
-
- d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
- This is called when a path-walk ends at dentry that was not acquired by
- doing a lookup in the parent directory. This includes "/", "." and "..",
- as well as procfs-style symlinks and mountpoint traversal.
-
- In this case, we are less concerned with whether the dentry is still
- fully correct, but rather that the inode is still valid. As with
- d_revalidate, most local filesystems will set this to NULL since their
- dcache entries are always valid.
-
- This function has the same return code semantics as d_revalidate.
-
- d_weak_revalidate is only called after leaving rcu-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.
-
- 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 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.
- Should not dereference pointers outside the dentry 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.
- ->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_init: called when a dentry is allocated
-
- 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.
-
- 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);
- }
-
- 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' while 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. -EISDIR may also be returned to tell pathwalk to
- ignore d_automount or any mounts.
-
- This function is only used if DCACHE_MANAGE_TRANSIT is set on the
- dentry being transited from.
-
- d_real: overlay/union type filesystems implement this method to return one of
- the underlying dentries hidden by the overlay. It is used in two
- different modes:
-
- Called from file_dentry() it returns the real dentry matching the inode
- argument. The real dentry may be from a lower layer already copied up,
- but still referenced from the file. This mode is selected with a
- non-NULL inode argument.
-
- With NULL inode the topmost real underlying dentry is returned.
-
-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.
-
-
-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>
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