Documentation/filesystems/vfs.rst

16 The Virtual File System (also known as the Virtual Filesystem Switch) is
17 the software layer in the kernel that provides the filesystem interface
19 kernel which allows different filesystem implementations to coexist.
22 are called from a process context.  Filesystem locking is described in
47 filesystem objects such as regular files, directories, FIFOs and other
56 filesystem implementation that the inode lives in.  Once the VFS has the
71 specific filesystem implementation can do its work.  You can see that
82 Registering and Mounting a Filesystem
85 To register and unregister a filesystem, use the following API
95 The passed struct file_system_type describes your filesystem.  When a
96 request is made to mount a filesystem onto a directory in your
98 specific filesystem.  New vfsmount referring to the tree returned by
110 This describes the filesystem.  The following
139 	the name of the filesystem type, such as "ext2", "iso9660",
147 	filesystem-specific data.
150 	Pointer to the array of filesystem parameters descriptors
155 	the method to call when a new instance of this filesystem should
159 	the method to call when an instance of this filesystem should be
171 	for internal VFS use: hlist of filesystem instances (superblocks)
179 	describes the filesystem, partly initialized by the specific
180 	filesystem code
197 on filesystem type.  E.g. for block filesystems, dev_name is interpreted
199 suitable filesystem image the method creates and initializes struct
202 ->mount() may choose to return a subtree of existing filesystem - it
209 super_operations" which describes the next level of the filesystem
212 Usually, a filesystem uses one of the generic mount() implementations
216 	mount a filesystem residing on a block device
219 	mount a filesystem that is not backed by a device
222 	mount a filesystem which shares the instance between all mounts
241 A superblock object represents a mounted filesystem.
248 filesystem.  The following members are defined:
329 	This method should be either NULL (normal UNIX filesystem
357 	down_write(&sb->s_umount). If filesystem implements it and wants
362 	called when VFS is locking a filesystem and forcing it into a
367 	called when VFS is unlocking a filesystem and making it writable
371 	called when VFS is unlocking a filesystem and making it writable
375 	called when the VFS needs to get filesystem statistics.
378 	called when the filesystem is remounted.  This is called with
382 	called when the VFS is unmounting a filesystem.
396 	the mount root dentry path relative to the filesystem root.
400 	filesystem-specific mount statistics.
403 	called by the VFS to read from filesystem quota file.
406 	called by the VFS to write to filesystem quota file.
413 	called by the sb cache shrinking function for the filesystem to
418 	called by the sb cache shrinking function for the filesystem to
420 	Optional, but any filesystem implementing this method needs to
424 	We can't do anything with any errors that the filesystem might
473 When none of the xattr handlers of a filesystem match the specified
474 attribute name or when a filesystem doesn't support extended attributes,
481 An inode object represents an object within the filesystem.
487 This describes how the VFS can manipulate an inode in your filesystem.
582 	The filesystem must return -EINVAL for any unsupported or
607 	If the filesystem stores the symlink target in ->i_link, the
622 	filesystem.
625 	rcu-walk mode, the filesystem must check the permission without
651 	method the filesystem can look up, possibly create and open the
654 	symlink, device, or just something filesystem won't do atomic
683         filesystem must define this operation to use
798 cache in your filesystem.  The following members are defined:
862 	be marked uptodate.  The filesystem should unlock the folio
864 	The filesystem does not need to modify the refcount on the folio;
875 	If the filesystem cannot perform the read at this time, it can
911 	filesystem should attempt to read all sync pages but may decide
919 	Called by the generic buffered write code to ask the filesystem
928 	The filesystem must return the locked pagecache folio for the
946 	The filesystem must take care of unlocking the folio,
957 	system does not go through the filesystem but instead uses bmap
976 	filesystem that the folio is about to be freed.  ->release_folio
987 	filesystem explicitly requesting it as nfs and 9p do (when they
989 	invalidate_inode_pages2().  If the filesystem makes such a call,
1032 	problems.  This callback allows a filesystem to indicate to the
1227 filesystem in which the inode resides.  When opening a device node
1230 driver information.  These support routines replace the filesystem file
1233 in the filesystem eventually ends up calling the device driver open()
1244 This describes how a filesystem can overload the standard dentry
1246 individual filesystem implementations.  Device drivers have no business
1281 	LOOKUP_RCU).  If in rcu-walk mode, the filesystem must
1328 	and inodes won't disappear, neither will our sb or filesystem
1401 	called to allow the filesystem to manage the transition from a
1438 There are a number of functions defined which permit a filesystem to
1491 On mount and remount the filesystem is passed a string containing a
1506 If a filesystem accepts mount options, it must define show_options() to