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 .. 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.rst.
 
 
 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_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:
 
 .. 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 when an inode is marked dirty.
         This is specifically for the inode itself being marked dirty,
         not its data.  If the update needs to be persisted by fdatasync(),
         then I_DIRTY_DATASYNC will be set in the flags argument.
 
 ``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
         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 user_namespace *, 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 user_namespace *, struct inode *,struct dentry *,const char *);
                 int (*mkdir) (struct user_namespace *, struct inode *,struct dentry *,umode_t);
                 int (*rmdir) (struct inode *,struct dentry *);
                 int (*mknod) (struct user_namespace *, struct inode *,struct dentry *,umode_t,dev_t);
                 int (*rename) (struct user_namespace *, 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 user_namespace *, struct inode *, int);
                 struct posix_acl * (*get_acl)(struct inode *, int, bool);
                 int (*setattr) (struct user_namespace *, struct dentry *, struct iattr *);
                 int (*getattr) (struct user_namespace *, 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 user_namespace *, struct inode *, struct dentry *, umode_t);
                 int (*set_acl)(struct user_namespace *, struct inode *, struct posix_acl *, int);
                 int (*fileattr_set)(struct user_namespace *mnt_userns,
                                     struct dentry *dentry, struct fileattr *fa);
                 int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
         };
 
 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.
 
 ``fileattr_get``
         called on ioctl(FS_IOC_GETFLAGS) and ioctl(FS_IOC_FSGETXATTR) to
         retrieve miscellaneous file flags and attributes.  Also called
         before the relevant SET operation to check what is being changed
         (in this case with i_rwsem locked exclusive).  If unset, then
         fall back to f_op->ioctl().
 
 ``fileattr_set``
         called on ioctl(FS_IOC_SETFLAGS) and ioctl(FS_IOC_FSSETXATTR) to
         change miscellaneous file flags and attributes.  Callers hold
         i_rwsem exclusive.  If unset, then fall back to f_op->ioctl().
 
 
 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 ->release_folio on clean folios with the private
 flag 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 'read_folio'.  The write
 process is more complicated and uses write_begin/write_end or
 dirty_folio 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 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 (*read_folio)(struct file *, struct folio *);
                 int (*writepages)(struct address_space *, struct writeback_control *);
                 bool (*dirty_folio)(struct address_space *, struct folio *);
                 void (*readahead)(struct readahead_control *);
                 int (*write_begin)(struct file *, struct address_space *mapping,
                                    loff_t pos, unsigned len,
                                 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 (*invalidate_folio) (struct folio *, size_t start, size_t len);
                 bool (*release_folio)(struct folio *, gfp_t);
                 void (*free_folio)(struct folio *);
                 ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
                 int (*migrate_folio)(struct mapping *, struct folio *dst,
                                 struct folio *src, enum migrate_mode);
                 int (*launder_folio) (struct folio *);
 
                 bool (*is_partially_uptodate) (struct folio *, size_t from,
                                                size_t count);
                 void (*is_dirty_writeback)(struct folio *, bool *, bool *);
                 int (*error_remove_page) (struct mapping *mapping, struct page *page);
                 int (*swap_activate)(struct swap_info_struct *sis, struct file *f, sector_t *span)
                 int (*swap_deactivate)(struct file *);
                 int (*swap_rw)(struct kiocb *iocb, struct iov_iter *iter);
         };
 
 ``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.
 
 ``read_folio``
         Called by the page cache to read a folio from the backing store.
         The 'file' argument supplies authentication information to network
         filesystems, and is generally not used by block based filesystems.
         It may be NULL if the caller does not have an open file (eg if
         the kernel is performing a read for itself rather than on behalf
         of a userspace process with an open file).
 
         If the mapping does not support large folios, the folio will
         contain a single page.  The folio will be locked when read_folio
         is called.  If the read completes successfully, the folio should
         be marked uptodate.  The filesystem should unlock the folio
         once the read has completed, whether it was successful or not.
         The filesystem does not need to modify the refcount on the folio;
         the page cache holds a reference count and that will not be
         released until the folio is unlocked.
 
         Filesystems may implement ->read_folio() synchronously.
         In normal operation, folios are read through the ->readahead()
         method.  Only if this fails, or if the caller needs to wait for
         the read to complete will the page cache call ->read_folio().
         Filesystems should not attempt to perform their own readahead
         in the ->read_folio() operation.
 
         If the filesystem cannot perform the read at this time, it can
         unlock the folio, do whatever action it needs to ensure that the
         read will succeed in the future and return AOP_TRUNCATED_PAGE.
         In this case, the caller should look up the folio, lock it,
         and call ->read_folio again.
 
         Callers may invoke the ->read_folio() method directly, but using
         read_mapping_folio() will take care of locking, waiting for the
         read to complete and handle cases such as AOP_TRUNCATED_PAGE.
 
 ``writepages``
         called by the VM to write out pages associated with the
         address_space object.  If wbc->sync_mode is WB_SYNC_ALL, then
         the writeback_control will specify a range of pages that must be
         written out.  If it is WB_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.
 
 ``dirty_folio``
         called by the VM to mark a folio as dirty.  This is particularly
         needed if an address space attaches private data to a folio, and
         that data needs to be updated when a folio is dirtied.  This is
         called, for example, when a memory mapped page gets modified.
         If defined, it should set the folio dirty flag, and the
         PAGECACHE_TAG_DIRTY search mark in i_pages.
 
 ``readahead``
         Called by the VM to read pages associated with the address_space
         object.  The pages are consecutive in the page cache and are
         locked.  The implementation should decrement the page refcount
         after starting I/O on each page.  Usually the page will be
         unlocked by the I/O completion handler.  The set of pages are
         divided into some sync pages followed by some async pages,
         rac->ra->async_size gives the number of async pages.  The
         filesystem should attempt to read all sync pages but may decide
         to stop once it reaches the async pages.  If it does decide to
         stop attempting I/O, it can simply return.  The caller will
         remove the remaining pages from the address space, unlock them
         and decrement the page refcount.  Set PageUptodate if the I/O
         completes successfully.  Setting PageError on any page will be
         ignored; simply unlock the page if an I/O error occurs.
 
 ``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).
 
         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.
 
 ``invalidate_folio``
         If a folio has private data, then invalidate_folio will be
         called when part or all of the folio 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 folio_size()).  Any private data associated with the folio
         should be updated to reflect this truncation.  If offset is 0
         and length is folio_size(), then the private data should be
         released, because the folio must be able to be completely
         discarded.  This may be done by calling the ->release_folio
         function, but in this case the release MUST succeed.
 
 ``release_folio``
         release_folio is called on folios with private data to tell the
         filesystem that the folio is about to be freed.  ->release_folio
         should remove any private data from the folio and clear the
         private flag.  If release_folio() fails, it should return false.
         release_folio() is used in two distinct though related cases.
         The first is when the VM wants to free a clean folio with no
         active users.  If ->release_folio succeeds, the folio will be
         removed from the address_space and be freed.
 
         The second case is when a request has been made to invalidate
         some or all folios 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 9p 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 folios are invalidated, then
         its release_folio will need to ensure this.  Possibly it can
         clear the uptodate flag if it cannot free private data yet.
 
 ``free_folio``
         free_folio is called once the folio 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.
 
 ``migrate_folio``
         This is used to compact the physical memory usage.  If the VM
         wants to relocate a folio (maybe from a memory device that is
         signalling imminent failure) it will pass a new folio and an old
         folio to this function.  migrate_folio should transfer any private
         data across and update any references that it has to the folio.
 
 ``launder_folio``
         Called before freeing a folio - it writes back the dirty folio.
         To prevent redirtying the folio, 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 is smaller than the size of the folio.
         If the required block is up to date then the read can complete
         without needing I/O to bring the whole page up to date.
 
 ``is_dirty_writeback``
         Called by the VM when attempting to reclaim a folio.  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 folio_test_dirty and folio_test_writeback but
         some filesystems have more complex state (unstable folios 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 folio 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 to prepare the given file for swap.  It should perform
         any validation and preparation necessary to ensure that writes
         can be performed with minimal memory allocation.  It should call
         add_swap_extent(), or the helper iomap_swapfile_activate(), and
         return the number of extents added.  If IO should be submitted
         through ->swap_rw(), it should set SWP_FS_OPS, otherwise IO will
         be submitted directly to the block device ``sis->bdev``.
 
 ``swap_deactivate``
         Called during swapoff on files where swap_activate was
         successful.
 
 ``swap_rw``
         Called to read or write swap pages when SWP_FS_OPS is set.
 
 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_FILE_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
     <https://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
     <https://www.tldp.org/LDP/khg/HyperNews/get/fs/vfstour.html>
 
 A small trail through the Linux kernel by Andries Brouwer. 2001
     <https://www.win.tue.nl/~aeb/linux/vfs/trail.html>

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