Original author: Richard Gooch <rgooch@atnf.csiro.au>
- Last updated on August 25, 2005
+ Last updated on June 24, 2007.
Copyright (C) 1999 Richard Gooch
Copyright (C) 2005 Pekka Enberg
This file is released under the GPLv2.
-What is it?
-===========
+Introduction
+============
-The Virtual File System (otherwise 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.
+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.
-A Quick Look At How It Works
-============================
-In this section I'll briefly describe how things work, before
-launching into the details. I'll start with describing what happens
-when user programs open and manipulate files, and then look from the
-other view which is how a filesystem is supported and subsequently
-mounted.
-
-
-Opening a File
---------------
-
-The VFS implements the open(2), stat(2), chmod(2) and similar system
-calls. The pathname argument is used by the VFS to search through the
-directory entry cache (dentry cache or "dcache"). This provides a very
-fast look-up mechanism to translate a pathname (filename) into a
-specific dentry.
-
-An individual dentry usually has a pointer to an inode. Inodes are the
-things that live on disc drives, and can be regular files (you know:
-those things that you write data into), directories, FIFOs and other
-beasts. Dentries live in RAM and are never saved to disc: they exist
-only for performance. Inodes live on disc and are copied into memory
-when required. Later any changes are written back to disc. The inode
-that lives in RAM is a VFS inode, and it is this which the dentry
-points to. A single inode can be pointed to by multiple dentries
-(think about hardlinks).
-
-The dcache is meant to be a view into your entire filespace. Unlike
-Linus, most of us losers can't fit enough dentries into RAM to cover
-all of our filespace, so the dcache has bits 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.
-
-To look up an inode (usually read from disc) 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. There will be more on this later.
+Directory Entry Cache (dcache)
+------------------------------
-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 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 it's 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.
+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
-function to do whatever is required.
-
-For as long as the file is open, it keeps the dentry "open" (in use),
-which in turn means that the VFS inode is still in use.
-
-All VFS system calls (i.e. open(2), stat(2), read(2), write(2),
-chmod(2) and so on) are called from a process context. You should
-assume that these calls are made without any kernel locks being
-held. This means that the processes may be executing the same piece of
-filesystem or driver code at the same time, on different
-processors. You should ensure that access to shared resources is
-protected by appropriate locks.
+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
--------------------------------------
+=====================================
-If you want to support a new kind of filesystem in the kernel, all you
-need to do is call register_filesystem(). You pass a structure
-describing the filesystem implementation (struct file_system_type)
-which is then added to an internal table of supported filesystems. You
-can do:
+To register and unregister a filesystem, use the following API
+functions:
-% cat /proc/filesystems
+ #include <linux/fs.h>
-to see what filesystems are currently available on your system.
+ extern int register_filesystem(struct file_system_type *);
+ extern int unregister_filesystem(struct file_system_type *);
-When a request is made to mount a block device onto a directory in
-your filespace the VFS will call the appropriate method for the
-specific filesystem. The dentry for the mount point will then be
-updated to point to the root inode for the new filesystem.
+The passed struct file_system_type describes your filesystem. When a
+request is made to mount a device onto a directory in your filespace,
+the VFS will call the appropriate get_sb() method for the specific
+filesystem. The dentry for the mount point will then be updated to
+point to the root inode for the new filesystem.
-It's now time to look at things in more detail.
+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 2.6.13, the following
+This describes the filesystem. As of kernel 2.6.22, the following
members are defined:
struct file_system_type {
const char *name;
int fs_flags;
- struct super_block *(*get_sb) (struct file_system_type *, int,
- const char *, void *);
+ int (*get_sb) (struct file_system_type *, int,
+ const char *, void *, struct vfsmount *);
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",
next: for internal VFS use: you should initialize this to NULL
+ s_lock_key, s_umount_key: lockdep-specific
+
The get_sb() method has the following arguments:
- struct super_block *sb: the superblock structure. This is partially
- initialized by the VFS and the rest must be initialized by the
- get_sb() method
+ 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
+ string (see "Mount Options" section)
- int silent: whether or not to be silent on error
+ struct vfsmount *mnt: a vfs-internal representation of a mount point
The get_sb() method must determine if the block device specified
-in the superblock contains a filesystem of the type the method
-supports. On success the method returns the superblock pointer, on
-failure it returns NULL.
+in the dev_name and fs_type contains a filesystem of the type the method
+supports. If it succeeds in opening the named block device, it initializes a
+struct super_block descriptor for the filesystem contained by the block device.
+On failure it returns an error.
The most interesting member of the superblock structure that the
get_sb() 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 generic one of the generic get_sb()
-implementations and provides a fill_super() method instead. The
-generic methods are:
+Usually, a filesystem uses one of the generic get_sb() implementations
+and provides a fill_super() method instead. The generic methods are:
get_sb_bdev: mount a filesystem residing on a block device
must initialize this properly.
void *data: arbitrary mount options, usually comes as an ASCII
- string
+ 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 2.6.13, 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);
void (*destroy_inode)(struct inode *);
- void (*read_inode) (struct inode *);
-
void (*dirty_inode) (struct inode *);
int (*write_inode) (struct inode *, int);
- void (*put_inode) (struct inode *);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
void (*put_super) (struct super_block *);
void (*write_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
- void (*write_super_lockfs) (struct super_block *);
- void (*unlockfs) (struct super_block *);
- int (*statfs) (struct super_block *, struct kstatfs *);
+ 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 *);
- void (*sync_inodes) (struct super_block *sb,
- struct writeback_control *wbc);
int (*show_options)(struct seq_file *, struct vfsmount *);
ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
or bottom half).
alloc_inode: this method is called by inode_alloc() to allocate memory
- for struct inode and initialize it.
+ 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.
-
- read_inode: this method is called to read a specific inode from the
- mounted filesystem. The i_ino member in the struct inode is
- initialized by the VFS to indicate which inode to read. Other
- members are filled in by this method.
-
- You can set this to NULL and use iget5_locked() instead of iget()
- to read inodes. This is necessary for filesystems for which the
- inode number is not sufficient to identify an inode.
+ 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.
inode to disc. The second parameter indicates whether the write
should be synchronous or not, not all filesystems check this flag.
- put_inode: called when the VFS inode is removed from the inode
- cache.
-
drop_inode: called when the last access to the inode is dropped,
with the inode_lock spinlock held.
a superblock. The second parameter indicates whether the method
should wait until the write out has been completed. Optional.
- write_super_lockfs: called when VFS is locking a filesystem and forcing
- it into a consistent state. This function is currently used by the
- Logical Volume Manager (LVM).
+ 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).
- unlockfs: called when VFS is unlocking a filesystem and making it writable
+ unfreeze_fs: called when VFS is unlocking a filesystem and making it writable
again.
- statfs: called when the VFS needs to get filesystem statistics. This
- is called with the kernel lock held
+ 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
umount_begin: called when the VFS is unmounting a filesystem.
- sync_inodes: called when the VFS is writing out dirty data associated with
- a superblock.
-
- show_options: called by the VFS to show mount options for /proc/<pid>/mounts.
+ 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.
-The read_inode() method 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.
+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.
+
+
+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 2.6.13, 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 *,int, struct nameidata *);
ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
ssize_t (*listxattr) (struct dentry *, char *, size_t);
int (*removexattr) (struct dentry *, const char *);
+ void (*truncate_range)(struct inode *, loff_t, loff_t);
};
Again, all methods are called without any locks being held, unless
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.
+
readlink: called by the readlink(2) system call. Only required if
you want to support reading symbolic links
follow_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 function returns a void pointer cookie
+ symbolic links. This method returns a void pointer cookie
that is passed to put_link().
put_link: called by the VFS to release resources allocated by
- follow_link(). The cookie returned by follow_link() is passed to
- to this function as the last parameter. It is used by filesystems
- such as NFS where page cache is not stable (i.e. page that was
- installed when the symbolic link walk started might not be in the
- page cache at the end of the walk).
-
- truncate: called by the VFS to change the size of a file. The i_size
- field of the inode is set to the desired size by the VFS before
- this function is called. This function is called by the truncate(2)
- system call and related functionality.
+ follow_link(). The cookie returned by follow_link() is passed
+ to this method as the last parameter. It is used by
+ filesystems such as NFS where page cache is not stable
+ (i.e. page that was installed when the symbolic link walk
+ started might not be in the page cache at the end of the
+ walk).
+
+ truncate: called by the VFS to change the size of a file. The
+ i_size field of the inode is set to the desired size by the
+ VFS before this method is called. This method is called by
+ the truncate(2) system call and related functionality.
permission: called by the VFS to check for access rights on a POSIX-like
filesystem.
- setattr: called by the VFS to set attributes for a file. This function is
- called by chmod(2) and related system calls.
+ 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 function is
- called by stat(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.
setxattr: called by the VFS to set an extended attribute for a file.
- Extended attribute is a name:value pair associated with an inode. This
- function is called by setxattr(2) system call.
-
- getxattr: called by the VFS to retrieve the value of an extended attribute
- name. This function is called by getxattr(2) function call.
-
- listxattr: called by the VFS to list all extended attributes for a given
- file. This function is called by listxattr(2) system call.
-
- removexattr: called by the VFS to remove an extended attribute from a file.
- This function is called by removexattr(2) system call.
-
+ Extended attribute is a name:value pair associated with an
+ inode. This method is called by setxattr(2) system call.
+
+ getxattr: called by the VFS to retrieve the value of an extended
+ attribute name. This method is called by getxattr(2) function
+ call.
+
+ listxattr: called by the VFS to list all extended attributes for a
+ given file. This method is called by listxattr(2) system call.
+
+ removexattr: called by the VFS to remove an extended attribute from
+ a file. This method is called by removexattr(2) system call.
+
+ truncate_range: a method provided by the underlying filesystem to truncate a
+ range of blocks , i.e. punch a hole somewhere in a file.
+
+
+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. While waiting ->sync_page (if defined) will be called on
+each page that is found to require writeback.
+
+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,
+sync_page, 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...
struct address_space_operations
-===============================
+-------------------------------
This describes how the VFS can manipulate mapping of a file to page cache in
-your filesystem. As of kernel 2.6.13, the following members are defined:
+your filesystem. As of kernel 2.6.22, the following members are defined:
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*set_page_dirty)(struct page *page);
int (*readpages)(struct file *filp, struct address_space *mapping,
struct list_head *pages, unsigned nr_pages);
- int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
- int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
+ 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);
int (*invalidatepage) (struct page *, unsigned long);
int (*releasepage) (struct page *, int);
loff_t offset, unsigned long nr_segs);
struct page* (*get_xip_page)(struct address_space *, sector_t,
int);
+ /* migrate the contents of a page to the specified target */
+ int (*migratepage) (struct page *, struct page *);
+ int (*launder_page) (struct page *);
+ int (*error_remove_page) (struct mapping *mapping, struct page *page);
};
- writepage: called by the VM write a dirty page to backing store.
+ 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.
sync_page: called by the VM to notify the backing store to perform all
queued I/O operations for a page. I/O operations for other pages
associated with this address_space object may also be performed.
+ This function is optional and is called only for pages with
+ PG_Writeback set while waiting for the writeback to complete.
+
writepages: called by the VM to write out pages associated with the
- address_space object.
+ 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.
+ 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.
- prepare_write: called by the generic write path in VM to set up a write
- request for a page.
+ 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.
- commit_write: called by the generic write path in VM to write page to
- its backing store.
+ The filesystem must return the locked pagecache page for the specified
+ offset, in *pagep, for the caller to write into.
- bmap: called by the VFS to map a logical block offset within object to
- physical block number. This method is use by for the legacy FIBMAP
- ioctl. Other uses are discouraged.
+ 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).
- invalidatepage: called by the VM on truncate to disassociate a page from its
- address_space mapping.
+ flags is a field for AOP_FLAG_xxx flags, described in
+ include/linux/fs.h.
- releasepage: called by the VFS to release filesystem specific metadata from
- a page.
+ A void * may be returned in fsdata, which then gets passed into
+ write_end.
- direct_IO: called by the VM for direct I/O writes and reads.
+ 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 (copied == len is always true
+ if write_begin was called with the AOP_FLAG_UNINTERRUPTIBLE flag).
+
+ 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 or a complete invalidation of the address space
+ (in the latter case 'offset' will always be 0).
+ Any private data associated with the page should be updated
+ to reflect this truncation. If offset is 0, 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. It may also remove the page from the
+ address_space. If this fails for some reason, it may indicate
+ failure with a 0 return value.
+ This 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 fadvice(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.
+
+ 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.
get_xip_page: called by the VM to translate a block number to a page.
The page is valid until the corresponding filesystem is unmounted.
Filesystems that want to use execute-in-place (XIP) need to implement
it. An example implementation can be found in fs/ext2/xip.c.
+ 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.
+
+ 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.
+
+ 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.
+
+
+The File Object
+===============
+
+A file object represents a file opened by a process.
+
struct file_operations
-======================
+----------------------
This describes how the VFS can manipulate an open file. As of kernel
-2.6.13, the following members are defined:
+2.6.22, 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 (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
- ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);
+ ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
+ ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
int (*readdir) (struct file *, void *, filldir_t);
unsigned int (*poll) (struct file *, struct poll_table_struct *);
int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
int (*release) (struct inode *, struct file *);
- int (*fsync) (struct file *, struct dentry *, int datasync);
+ int (*fsync) (struct file *, int datasync);
int (*aio_fsync) (struct kiocb *, 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 (*dir_notify)(struct file *filp, unsigned long arg);
int (*flock) (struct file *, int, struct file_lock *);
+ ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, size_t, unsigned int);
+ ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);
};
Again, all methods are called without any locks being held, unless
check_flags: called by the fcntl(2) system call for F_SETFL command
- dir_notify: called by the fcntl(2) system call for F_NOTIFY 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
+
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
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 2.6.13, the following members are
+the VFS uses a default. As of kernel 2.6.22, the following members are
defined:
struct dentry_operations {
int (*d_delete)(struct dentry *);
void (*d_release)(struct dentry *);
void (*d_iput)(struct dentry *, struct inode *);
+ char *(*d_dname)(struct dentry *, char *, int);
};
d_revalidate: called when the VFS needs to revalidate a dentry. This
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);
+}
+
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 APIs
+Directory Entry Cache API
--------------------------
There are a number of functions defined which permit a filesystem to
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 d_put()
+ and the dentry is returned. The caller must use dput()
to free the dentry when it finishes using it.
+For further information on dentry locking, please refer to the document
+Documentation/filesystems/dentry-locking.txt.
-RCU-based dcache locking model
-------------------------------
+Mount Options
+=============
-On many workloads, the most common operation on dcache is
-to look up a dentry, given a parent dentry and the name
-of the child. Typically, for every open(), stat() etc.,
-the dentry corresponding to the pathname will be looked
-up by walking the tree starting with the first component
-of the pathname and using that dentry along with the next
-component to look up the next level and so on. Since it
-is a frequent operation for workloads like multiuser
-environments and web servers, it is important to optimize
-this path.
-
-Prior to 2.5.10, dcache_lock was acquired in d_lookup and thus
-in every component during path look-up. Since 2.5.10 onwards,
-fast-walk algorithm changed this by holding the dcache_lock
-at the beginning and walking as many cached path component
-dentries as possible. This significantly decreases the number
-of acquisition of dcache_lock. However it also increases the
-lock hold time significantly and affects performance in large
-SMP machines. Since 2.5.62 kernel, dcache has been using
-a new locking model that uses RCU to make dcache look-up
-lock-free.
-
-The current dcache locking model is not very different from the existing
-dcache locking model. Prior to 2.5.62 kernel, dcache_lock
-protected the hash chain, d_child, d_alias, d_lru lists as well
-as d_inode and several other things like mount look-up. RCU-based
-changes affect only the way the hash chain is protected. For everything
-else the dcache_lock must be taken for both traversing as well as
-updating. The hash chain updates too take the dcache_lock.
-The significant change is the way d_lookup traverses the hash chain,
-it doesn't acquire the dcache_lock for this and rely on RCU to
-ensure that the dentry has not been *freed*.
-
-
-Dcache locking details
-----------------------
+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.
+
+A simple method of saving options at mount/remount time and showing
+them is provided with the save_mount_options() and
+generic_show_options() helper functions. Please note, that using
+these may have drawbacks. For more info see header comments for these
+functions in fs/namespace.c.
+
+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>
-For many multi-user workloads, open() and stat() on files are
-very frequently occurring operations. Both involve walking
-of path names to find the dentry corresponding to the
-concerned file. In 2.4 kernel, dcache_lock was held
-during look-up of each path component. Contention and
-cache-line bouncing of this global lock caused significant
-scalability problems. With the introduction of RCU
-in Linux kernel, this was worked around by making
-the look-up of path components during path walking lock-free.
-
-
-Safe lock-free look-up of dcache hash table
-===========================================
-
-Dcache is a complex data structure with the hash table entries
-also linked together in other lists. In 2.4 kernel, dcache_lock
-protected all the lists. We applied RCU only on hash chain
-walking. The rest of the lists are still protected by dcache_lock.
-Some of the important changes are :
-
-1. The deletion from hash chain is done using hlist_del_rcu() macro which
- doesn't initialize next pointer of the deleted dentry and this
- allows us to walk safely lock-free while a deletion is happening.
-
-2. Insertion of a dentry into the hash table is done using
- hlist_add_head_rcu() which take care of ordering the writes -
- the writes to the dentry must be visible before the dentry
- is inserted. This works in conjunction with hlist_for_each_rcu()
- while walking the hash chain. The only requirement is that
- all initialization to the dentry must be done before hlist_add_head_rcu()
- since we don't have dcache_lock protection while traversing
- the hash chain. This isn't different from the existing code.
-
-3. The dentry looked up without holding dcache_lock by cannot be
- returned for walking if it is unhashed. It then may have a NULL
- d_inode or other bogosity since RCU doesn't protect the other
- fields in the dentry. We therefore use a flag DCACHE_UNHASHED to
- indicate unhashed dentries and use this in conjunction with a
- per-dentry lock (d_lock). Once looked up without the dcache_lock,
- we acquire the per-dentry lock (d_lock) and check if the
- dentry is unhashed. If so, the look-up is failed. If not, the
- reference count of the dentry is increased and the dentry is returned.
-
-4. Once a dentry is looked up, it must be ensured during the path
- walk for that component it doesn't go away. In pre-2.5.10 code,
- this was done holding a reference to the dentry. dcache_rcu does
- the same. In some sense, dcache_rcu path walking looks like
- the pre-2.5.10 version.
-
-5. All dentry hash chain updates must take the dcache_lock as well as
- the per-dentry lock in that order. dput() does this to ensure
- that a dentry that has just been looked up in another CPU
- doesn't get deleted before dget() can be done on it.
-
-6. There are several ways to do reference counting of RCU protected
- objects. One such example is in ipv4 route cache where
- deferred freeing (using call_rcu()) is done as soon as
- the reference count goes to zero. This cannot be done in
- the case of dentries because tearing down of dentries
- require blocking (dentry_iput()) which isn't supported from
- RCU callbacks. Instead, tearing down of dentries happen
- synchronously in dput(), but actual freeing happens later
- when RCU grace period is over. This allows safe lock-free
- walking of the hash chains, but a matched dentry may have
- been partially torn down. The checking of DCACHE_UNHASHED
- flag with d_lock held detects such dentries and prevents
- them from being returned from look-up.
-
-
-Maintaining POSIX rename semantics
-==================================
-
-Since look-up of dentries is lock-free, it can race against
-a concurrent rename operation. For example, during rename
-of file A to B, look-up of either A or B must succeed.
-So, if look-up of B happens after A has been removed from the
-hash chain but not added to the new hash chain, it may fail.
-Also, a comparison while the name is being written concurrently
-by a rename may result in false positive matches violating
-rename semantics. Issues related to race with rename are
-handled as described below :
-
-1. Look-up can be done in two ways - d_lookup() which is safe
- from simultaneous renames and __d_lookup() which is not.
- If __d_lookup() fails, it must be followed up by a d_lookup()
- to correctly determine whether a dentry is in the hash table
- or not. d_lookup() protects look-ups using a sequence
- lock (rename_lock).
-
-2. The name associated with a dentry (d_name) may be changed if
- a rename is allowed to happen simultaneously. To avoid memcmp()
- in __d_lookup() go out of bounds due to a rename and false
- positive comparison, the name comparison is done while holding the
- per-dentry lock. This prevents concurrent renames during this
- operation.
-
-3. Hash table walking during look-up may move to a different bucket as
- the current dentry is moved to a different bucket due to rename.
- But we use hlists in dcache hash table and they are null-terminated.
- So, even if a dentry moves to a different bucket, hash chain
- walk will terminate. [with a list_head list, it may not since
- termination is when the list_head in the original bucket is reached].
- Since we redo the d_parent check and compare name while holding
- d_lock, lock-free look-up will not race against d_move().
-
-4. There can be a theoretical race when a dentry keeps coming back
- to original bucket due to double moves. Due to this look-up may
- consider that it has never moved and can end up in a infinite loop.
- But this is not any worse that theoretical livelocks we already
- have in the kernel.
-
-
-Important guidelines for filesystem developers related to dcache_rcu
-====================================================================
-
-1. Existing dcache interfaces (pre-2.5.62) exported to filesystem
- don't change. Only dcache internal implementation changes. However
- filesystems *must not* delete from the dentry hash chains directly
- using the list macros like allowed earlier. They must use dcache
- APIs like d_drop() or __d_drop() depending on the situation.
-
-2. d_flags is now protected by a per-dentry lock (d_lock). All
- access to d_flags must be protected by it.
-
-3. For a hashed dentry, checking of d_count needs to be protected
- by d_lock.
-
-
-Papers and other documentation on dcache locking
-================================================
-
-1. Scaling dcache with RCU (http://linuxjournal.com/article.php?sid=7124).
-
-2. http://lse.sourceforge.net/locking/dcache/dcache.html
+A small trail through the Linux kernel by Andries Brouwer. 2001
+ <http://www.win.tue.nl/~aeb/linux/vfs/trail.html>
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff