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diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst new file mode 100644 index 000000000000..0f85ab21c2ca --- /dev/null +++ b/Documentation/filesystems/vfs.rst @@ -0,0 +1,1428 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================================= +Overview of the Linux Virtual File System +========================================= + +Original author: Richard Gooch <rgooch@atnf.csiro.au> + +- Copyright (C) 1999 Richard Gooch +- Copyright (C) 2005 Pekka Enberg + + +Introduction +============ + +The Virtual File System (also known as the Virtual Filesystem Switch) is +the software layer in the kernel that provides the filesystem interface +to userspace programs. It also provides an abstraction within the +kernel which allows different filesystem implementations to coexist. + +VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on +are called from a process context. Filesystem locking is described in +the document Documentation/filesystems/Locking. + + +Directory Entry Cache (dcache) +------------------------------ + +The VFS implements the open(2), stat(2), chmod(2), and similar system +calls. The pathname argument that is passed to them is used by the VFS +to search through the directory entry cache (also known as the dentry +cache or dcache). This provides a very fast look-up mechanism to +translate a pathname (filename) into a specific dentry. Dentries live +in RAM and are never saved to disc: they exist only for performance. + +The dentry cache is meant to be a view into your entire filespace. As +most computers cannot fit all dentries in the RAM at the same time, some +bits of the cache are missing. In order to resolve your pathname into a +dentry, the VFS may have to resort to creating dentries along the way, +and then loading the inode. This is done by looking up the inode. + + +The Inode Object +---------------- + +An individual dentry usually has a pointer to an inode. Inodes are +filesystem objects such as regular files, directories, FIFOs and other +beasts. They live either on the disc (for block device filesystems) or +in the memory (for pseudo filesystems). Inodes that live on the disc +are copied into the memory when required and changes to the inode are +written back to disc. A single inode can be pointed to by multiple +dentries (hard links, for example, do this). + +To look up an inode requires that the VFS calls the lookup() method of +the parent directory inode. This method is installed by the specific +filesystem implementation that the inode lives in. Once the VFS has the +required dentry (and hence the inode), we can do all those boring things +like open(2) the file, or stat(2) it to peek at the inode data. The +stat(2) operation is fairly simple: once the VFS has the dentry, it +peeks at the inode data and passes some of it back to userspace. + + +The File Object +--------------- + +Opening a file requires another operation: allocation of a file +structure (this is the kernel-side implementation of file descriptors). +The freshly allocated file structure is initialized with a pointer to +the dentry and a set of file operation member functions. These are +taken from the inode data. The open() file method is then called so the +specific filesystem implementation can do its work. You can see that +this is another switch performed by the VFS. The file structure is +placed into the file descriptor table for the process. + +Reading, writing and closing files (and other assorted VFS operations) +is done by using the userspace file descriptor to grab the appropriate +file structure, and then calling the required file structure method to +do whatever is required. For as long as the file is open, it keeps the +dentry in use, which in turn means that the VFS inode is still in use. + + +Registering and Mounting a Filesystem +===================================== + +To register and unregister a filesystem, use the following API +functions: + +.. code-block:: c + + #include <linux/fs.h> + + extern int register_filesystem(struct file_system_type *); + extern int unregister_filesystem(struct file_system_type *); + +The passed struct file_system_type describes your filesystem. When a +request is made to mount a filesystem onto a directory in your +namespace, the VFS will call the appropriate mount() method for the +specific filesystem. New vfsmount referring to the tree returned by +->mount() will be attached to the mountpoint, so that when pathname +resolution reaches the mountpoint it will jump into the root of that +vfsmount. + +You can see all filesystems that are registered to the kernel in the +file /proc/filesystems. + + +struct file_system_type +----------------------- + +This describes the filesystem. As of kernel 2.6.39, the following +members are defined: + +.. code-block:: c + + struct file_system_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: + +.. code-block:: c + + struct super_operations { + struct inode *(*alloc_inode)(struct super_block *sb); + void (*destroy_inode)(struct inode *); + + void (*dirty_inode) (struct inode *, int flags); + int (*write_inode) (struct inode *, int); + void (*drop_inode) (struct inode *); + void (*delete_inode) (struct inode *); + void (*put_super) (struct super_block *); + int (*sync_fs)(struct super_block *sb, int wait); + int (*freeze_fs) (struct super_block *); + int (*unfreeze_fs) (struct super_block *); + int (*statfs) (struct dentry *, struct kstatfs *); + int (*remount_fs) (struct super_block *, int *, char *); + void (*clear_inode) (struct inode *); + void (*umount_begin) (struct super_block *); + + int (*show_options)(struct seq_file *, struct dentry *); + + ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t); + ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t); + int (*nr_cached_objects)(struct super_block *); + void (*free_cached_objects)(struct super_block *, int); + }; + +All methods are called without any locks being held, unless otherwise +noted. This means that most methods can block safely. All methods are +only called from a process context (i.e. not from an interrupt handler +or bottom half). + +``alloc_inode`` + this method is called by alloc_inode() to allocate memory for + struct inode and initialize it. If this function is not + defined, a simple 'struct inode' is allocated. Normally + alloc_inode will be used to allocate a larger structure which + contains a 'struct inode' embedded within it. + +``destroy_inode`` + this method is called by destroy_inode() to release resources + allocated for struct inode. It is only required if + ->alloc_inode was defined and simply undoes anything done by + ->alloc_inode. + +``dirty_inode`` + this method is called by the VFS 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: + +.. code-block:: c + + 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: + +.. code-block:: c + + 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: + +.. code-block:: c + + struct file_operations { + struct module *owner; + loff_t (*llseek) (struct file *, loff_t, int); + ssize_t (*read) (struct file *, char __user *, size_t, loff_t *); + ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *); + ssize_t (*read_iter) (struct kiocb *, struct iov_iter *); + ssize_t (*write_iter) (struct kiocb *, struct iov_iter *); + int (*iopoll)(struct kiocb *kiocb, bool spin); + int (*iterate) (struct file *, struct dir_context *); + int (*iterate_shared) (struct file *, struct dir_context *); + __poll_t (*poll) (struct file *, struct poll_table_struct *); + long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); + long (*compat_ioctl) (struct file *, unsigned int, unsigned long); + int (*mmap) (struct file *, struct vm_area_struct *); + int (*open) (struct inode *, struct file *); + int (*flush) (struct file *, fl_owner_t id); + int (*release) (struct inode *, struct file *); + int (*fsync) (struct file *, loff_t, loff_t, int datasync); + int (*fasync) (int, struct file *, int); + int (*lock) (struct file *, int, struct file_lock *); + ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int); + unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long); + int (*check_flags)(int); + int (*flock) (struct file *, int, struct file_lock *); + ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int); + ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int); + int (*setlease)(struct file *, long, struct file_lock **, void **); + long (*fallocate)(struct file *file, int mode, loff_t offset, + loff_t len); + void (*show_fdinfo)(struct seq_file *m, struct file *f); + #ifndef CONFIG_MMU + unsigned (*mmap_capabilities)(struct file *); + #endif + ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int); + loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in, + struct file *file_out, loff_t pos_out, + loff_t len, unsigned int remap_flags); + int (*fadvise)(struct file *, loff_t, loff_t, int); + }; + +Again, all methods are called without any locks being held, unless +otherwise noted. + +``llseek`` + called when the VFS needs to move the file position index + +``read`` + called by read(2) and related system calls + +``read_iter`` + possibly asynchronous read with iov_iter as destination + +``write`` + called by write(2) and related system calls + +``write_iter`` + possibly asynchronous write with iov_iter as source + +``iopoll`` + called when aio wants to poll for completions on HIPRI iocbs + +``iterate`` + called when the VFS needs to read the directory contents + +``iterate_shared`` + called when the VFS needs to read the directory contents when + filesystem supports concurrent dir iterators + +``poll`` + called by the VFS when a process wants to check if there is + activity on this file and (optionally) go to sleep until there + is activity. Called by the select(2) and poll(2) system calls + +``unlocked_ioctl`` + called by the ioctl(2) system call. + +``compat_ioctl`` + called by the ioctl(2) system call when 32 bit system calls are + used on 64 bit kernels. + +``mmap`` + called by the mmap(2) system call + +``open`` + called by the VFS when an inode should be opened. When the VFS + opens a file, it creates a new "struct file". It then calls the + open method for the newly allocated file structure. You might + think that the open method really belongs in "struct + inode_operations", and you may be right. I think it's done the + way it is because it makes filesystems simpler to implement. + The open() method is a good place to initialize the + "private_data" member in the file structure if you want to point + to a device structure + +``flush`` + called by the close(2) system call to flush a file + +``release`` + called when the last reference to an open file is closed + +``fsync`` + called by the fsync(2) system call. Also see the section above + entitled "Handling errors during writeback". + +``fasync`` + called by the fcntl(2) system call when asynchronous + (non-blocking) mode is enabled for a file + +``lock`` + called by the fcntl(2) system call for F_GETLK, F_SETLK, and + F_SETLKW commands + +``get_unmapped_area`` + called by the mmap(2) system call + +``check_flags`` + called by the fcntl(2) system call for F_SETFL command + +``flock`` + called by the flock(2) system call + +``splice_write`` + called by the VFS to splice data from a pipe to a file. This + method is used by the splice(2) system call + +``splice_read`` + called by the VFS to splice data from file to a pipe. This + method is used by the splice(2) system call + +``setlease`` + called by the VFS to set or release a file lock lease. setlease + implementations should call generic_setlease to record or remove + the lease in the inode after setting it. + +``fallocate`` + called by the VFS to preallocate blocks or punch a hole. + +``copy_file_range`` + called by the copy_file_range(2) system call. + +``remap_file_range`` + called by the ioctl(2) system call for FICLONERANGE and FICLONE + and FIDEDUPERANGE commands to remap file ranges. An + implementation should remap len bytes at pos_in of the source + file into the dest file at pos_out. Implementations must handle + callers passing in len == 0; this means "remap to the end of the + source file". The return value should the number of bytes + remapped, or the usual negative error code if errors occurred + before any bytes were remapped. The remap_flags parameter + accepts REMAP_FILE_* flags. If REMAP_FILE_DEDUP is set then the + implementation must only remap if the requested file ranges have + identical contents. If REMAP_CAN_SHORTEN is set, the caller is + ok with the implementation shortening the request length to + satisfy alignment or EOF requirements (or any other reason). + +``fadvise`` + possibly called by the fadvise64() system call. + +Note that the file operations are implemented by the specific +filesystem in which the inode resides. When opening a device node +(character or block special) most filesystems will call special +support routines in the VFS which will locate the required device +driver information. These support routines replace the filesystem file +operations with those for the device driver, and then proceed to call +the new open() method for the file. This is how opening a device file +in the filesystem eventually ends up calling the device driver open() +method. + + +Directory Entry Cache (dcache) +============================== + + +struct dentry_operations +------------------------ + +This describes how a filesystem can overload the standard dentry +operations. Dentries and the dcache are the domain of the VFS and the +individual filesystem implementations. Device drivers have no business +here. These methods may be set to NULL, as they are either optional or +the VFS uses a default. As of kernel 2.6.22, the following members are +defined: + +.. code-block:: c + + struct dentry_operations { + int (*d_revalidate)(struct dentry *, unsigned int); + int (*d_weak_revalidate)(struct dentry *, unsigned int); + int (*d_hash)(const struct dentry *, struct qstr *); + int (*d_compare)(const struct dentry *, + unsigned int, const char *, const struct qstr *); + int (*d_delete)(const struct dentry *); + int (*d_init)(struct dentry *); + void (*d_release)(struct dentry *); + void (*d_iput)(struct dentry *, struct inode *); + char *(*d_dname)(struct dentry *, char *, int); + struct vfsmount *(*d_automount)(struct path *); + int (*d_manage)(const struct path *, bool); + struct dentry *(*d_real)(struct dentry *, const struct inode *); + }; + +``d_revalidate`` + called when the VFS needs to revalidate a dentry. This is + called whenever a name look-up finds a dentry in the dcache. + Most local filesystems leave this as NULL, because all their + dentries in the dcache are valid. Network filesystems are + different since things can change on the server without the + client necessarily being aware of it. + + This function should return a positive value if the dentry is + still valid, and zero or a negative error code if it isn't. + + d_revalidate may be called in rcu-walk mode (flags & + LOOKUP_RCU). If in rcu-walk mode, the filesystem must + revalidate the dentry without blocking or storing to the dentry, + d_parent and d_inode should not be used without care (because + they can change and, in d_inode case, even become NULL under + us). + + If a situation is encountered that rcu-walk cannot handle, + return + -ECHILD and it will be called again in ref-walk mode. + +``_weak_revalidate`` + called when the VFS needs to revalidate a "jumped" dentry. This + is called when a path-walk ends at dentry that was not acquired + by doing a lookup in the parent directory. This includes "/", + "." and "..", as well as procfs-style symlinks and mountpoint + traversal. + + In this case, we are less concerned with whether the dentry is + still fully correct, but rather that the inode is still valid. + As with d_revalidate, most local filesystems will set this to + NULL since their dcache entries are always valid. + + This function has the same return code semantics as + d_revalidate. + + d_weak_revalidate is only called after leaving rcu-walk mode. + +``d_hash`` + called when the VFS adds a dentry to the hash table. The first + dentry passed to d_hash is the parent directory that the name is + to be hashed into. + + Same locking and synchronisation rules as d_compare regarding + what is safe to dereference etc. + +``d_compare`` + called to compare a dentry name with a given name. The first + dentry is the parent of the dentry to be compared, the second is + the child dentry. len and name string are properties of the + dentry to be compared. qstr is the name to compare it with. + + Must be constant and idempotent, and should not take locks if + possible, and should not or store into the dentry. Should not + dereference pointers outside the dentry without lots of care + (eg. d_parent, d_inode, d_name should not be used). + + However, our vfsmount is pinned, and RCU held, so the dentries + and inodes won't disappear, neither will our sb or filesystem + module. ->d_sb may be used. + + It is a tricky calling convention because it needs to be called + under "rcu-walk", ie. without any locks or references on things. + +``d_delete`` + called when the last reference to a dentry is dropped and the + dcache is deciding whether or not to cache it. Return 1 to + delete immediately, or 0 to cache the dentry. Default is NULL + which means to always cache a reachable dentry. d_delete must + be constant and idempotent. + +``d_init`` + called when a dentry is allocated + +``d_release`` + called when a dentry is really deallocated + +``d_iput`` + called when a dentry loses its inode (just prior to its being + deallocated). The default when this is NULL is that the VFS + calls iput(). If you define this method, you must call iput() + yourself + +``d_dname`` + called when the pathname of a dentry should be generated. + Useful for some pseudo filesystems (sockfs, pipefs, ...) to + delay pathname generation. (Instead of doing it when dentry is + created, it's done only when the path is needed.). Real + filesystems probably dont want to use it, because their dentries + are present in global dcache hash, so their hash should be an + invariant. As no lock is held, d_dname() should not try to + modify the dentry itself, unless appropriate SMP safety is used. + CAUTION : d_path() logic is quite tricky. The correct way to + return for example "Hello" is to put it at the end of the + buffer, and returns a pointer to the first char. + dynamic_dname() helper function is provided to take care of + this. + + Example : + +.. code-block:: c + + static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen) + { + return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]", + dentry->d_inode->i_ino); + } + +``d_automount`` + called when an automount dentry is to be traversed (optional). + This should create a new VFS mount record and return the record + to the caller. The caller is supplied with a path parameter + giving the automount directory to describe the automount target + and the parent VFS mount record to provide inheritable mount + parameters. NULL should be returned if someone else managed to + make the automount first. If the vfsmount creation failed, then + an error code should be returned. If -EISDIR is returned, then + the directory will be treated as an ordinary directory and + returned to pathwalk to continue walking. + + If a vfsmount is returned, the caller will attempt to mount it + on the mountpoint and will remove the vfsmount from its + expiration list in the case of failure. The vfsmount should be + returned with 2 refs on it to prevent automatic expiration - the + caller will clean up the additional ref. + + This function is only used if DCACHE_NEED_AUTOMOUNT is set on + the dentry. This is set by __d_instantiate() if S_AUTOMOUNT is + set on the inode being added. + +``d_manage`` + called to allow the filesystem to manage the transition from a + dentry (optional). This allows autofs, for example, to hold up + clients waiting to explore behind a 'mountpoint' while letting + the daemon go past and construct the subtree there. 0 should be + returned to let the calling process continue. -EISDIR can be + returned to tell pathwalk to use this directory as an ordinary + directory and to ignore anything mounted on it and not to check + the automount flag. Any other error code will abort pathwalk + completely. + + If the 'rcu_walk' parameter is true, then the caller is doing a + pathwalk in RCU-walk mode. Sleeping is not permitted in this + mode, and the caller can be asked to leave it and call again by + returning -ECHILD. -EISDIR may also be returned to tell + pathwalk to ignore d_automount or any mounts. + + This function is only used if DCACHE_MANAGE_TRANSIT is set on + the dentry being transited from. + +``d_real`` + overlay/union type filesystems implement this method to return + one of the underlying dentries hidden by the overlay. It is + used in two different modes: + + Called from file_dentry() it returns the real dentry matching + the inode argument. The real dentry may be from a lower layer + already copied up, but still referenced from the file. This + mode is selected with a non-NULL inode argument. + + With NULL inode the topmost real underlying dentry is returned. + +Each dentry has a pointer to its parent dentry, as well as a hash list +of child dentries. Child dentries are basically like files in a +directory. + + +Directory Entry Cache API +-------------------------- + +There are a number of functions defined which permit a filesystem to +manipulate dentries: + +``dget`` + open a new handle for an existing dentry (this just increments + the usage count) + +``dput`` + close a handle for a dentry (decrements the usage count). If + the usage count drops to 0, and the dentry is still in its + parent's hash, the "d_delete" method is called to check whether + it should be cached. If it should not be cached, or if the + dentry is not hashed, it is deleted. Otherwise cached dentries + are put into an LRU list to be reclaimed on memory shortage. + +``d_drop`` + this unhashes a dentry from its parents hash list. A subsequent + call to dput() will deallocate the dentry if its usage count + drops to 0 + +``d_delete`` + delete a dentry. If there are no other open references to the + dentry then the dentry is turned into a negative dentry (the + d_iput() method is called). If there are other references, then + d_drop() is called instead + +``d_add`` + add a dentry to its parents hash list and then calls + d_instantiate() + +``d_instantiate`` + add a dentry to the alias hash list for the inode and updates + the "d_inode" member. The "i_count" member in the inode + structure should be set/incremented. If the inode pointer is + NULL, the dentry is called a "negative dentry". This function + is commonly called when an inode is created for an existing + negative dentry + +``d_lookup`` + look up a dentry given its parent and path name component It + looks up the child of that given name from the dcache hash + table. If it is found, the reference count is incremented and + the dentry is returned. The caller must use dput() to free the + dentry when it finishes using it. + + +Mount Options +============= + + +Parsing options +--------------- + +On mount and remount the filesystem is passed a string containing a +comma separated list of mount options. The options can have either of +these forms: + + option + option=value + +The <linux/parser.h> header defines an API that helps parse these +options. There are plenty of examples on how to use it in existing +filesystems. + + +Showing options +--------------- + +If a filesystem accepts mount options, it must define show_options() to +show all the currently active options. The rules are: + + - options MUST be shown which are not default or their values differ + from the default + + - options MAY be shown which are enabled by default or have their + default value + +Options used only internally between a mount helper and the kernel (such +as file descriptors), or which only have an effect during the mounting +(such as ones controlling the creation of a journal) are exempt from the +above rules. + +The underlying reason for the above rules is to make sure, that a mount +can be accurately replicated (e.g. umounting and mounting again) based +on the information found in /proc/mounts. + + +Resources +========= + +(Note some of these resources are not up-to-date with the latest kernel + version.) + +Creating Linux virtual filesystems. 2002 + <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> |