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
+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
+on are called from a process context. Filesystem locking is described
in the document Documentation/filesystems/Locking.
------------------------------
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
+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
+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
+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
+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
+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
+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
+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
+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 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
+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.
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
+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
+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
+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.
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
+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()
struct file_system_type
-----------------------
-This describes the filesystem. As of kernel 2.6.39, the following
+This describes the filesystem. As of kernel 2.6.39, the following
members are defined:
struct file_system_type {
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
+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:
+and provides a fill_super() callback instead. The generic variants are:
mount_bdev: mount a filesystem residing on a block device
A fill_super() callback implementation has the following arguments:
- struct super_block *sb: the superblock structure. The callback
+ struct super_block *sb: the superblock structure. The callback
must initialize this properly.
void *data: arbitrary mount options, usually comes as an ASCII
-----------------------
This describes how the VFS can manipulate the superblock of your
-filesystem. As of kernel 2.6.22, the following members are defined:
+filesystem. As of kernel 2.6.22, the following members are defined:
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
};
All methods are called without any locks being held, unless otherwise
-noted. This means that most methods can block safely. All methods are
+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).
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
+ (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.
+ 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
statfs: called when the VFS needs to get filesystem statistics.
- remount_fs: called when the filesystem is remounted. This is called
+ 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
+ clear_inode: called then the VFS clears the inode. Optional
umount_begin: called when the VFS is unmounting a filesystem.
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
+ 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
+ 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
+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.
-----------------------
This describes how the VFS can manipulate an inode in your
-filesystem. As of kernel 2.6.22, the following members are defined:
+filesystem. As of kernel 2.6.22, the following members are defined:
struct inode_operations {
int (*create) (struct inode *,struct dentry *, umode_t, bool);
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
+ 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). 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
+ 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
+ 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
+ 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
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
+ 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
+ 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
+ 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
+ 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
+ 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
+ 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
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
+ 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
+ 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
+ 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.
+ 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
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
+PagePrivate set. Clean pages without PagePrivate and with no external
references will be released without notice being given to the
address_space.
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
+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.
-------------------------------
This describes how the VFS can manipulate mapping of a file to page cache in
-your filesystem. The following members are defined:
+your filesystem. The following members are defined:
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
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
+ 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
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
+ 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
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
+ 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
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
+ 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
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
+ 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
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
+ 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
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
+ 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.
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
+ 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
+ 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
+ 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
+ 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.
+ 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,
+ 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
The File Object
===============
-A file object represents a file opened by a process. This is also known
+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
+This describes how the VFS can manipulate an open file. As of kernel
4.18, the following members are defined:
struct file_operations {
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
+ is activity. Called by the select(2) and poll(2) system calls
unlocked_ioctl: called by the ioctl(2) system call.
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
+ 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
+ "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
+ 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
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
+ 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
flock: called by the flock(2) system call
- splice_write: called by the VFS to splice data from a pipe to a file. This
+ 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
+ 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
+ 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.
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
+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
+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
+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.
------------------------
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
+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 {
struct dentry *(*d_real)(struct dentry *, const struct inode *);
};
- d_revalidate: called when the VFS needs to revalidate a dentry. This
+ 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
+ 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.
d_weak_revalidate: called when the VFS needs to revalidate a "jumped" dentry.
This is called when a path-walk ends at dentry that was not acquired by
- doing a lookup in the parent directory. This includes "/", "." and "..",
+ 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
+ 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.
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
+ 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
+ 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.
+ 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.
"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
+ 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
+ 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
+ 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
+ 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
+ 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.
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
+of child dentries. Child dentries are basically like files in a
directory.
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
+ 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
+ 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
+ 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
+ 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
+ (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
+ 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
+ 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()
+ 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
projects / openwrt / staging / blogic.git / commitdiff