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:

#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_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:

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.

_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>