代码拉取完成，页面将自动刷新
Original author: Richard Gooch <rgooch@atnf.csiro.au>
The Virtual File System (also known as the Virtual Filesystem Switch) is the software layer in the kernel that provides the filesystem interface to userspace programs. It also provides an abstraction within the kernel which allows different filesystem implementations to coexist.
VFS system calls open(2), stat(2), read(2), write(2), chmod(2) and so on are called from a process context. Filesystem locking is described in the document Documentation/filesystems/locking.rst.
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.
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.
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.
To register and unregister a filesystem, use the following API functions:
#include <linux/fs.h>
extern int register_filesystem(struct file_system_type *);
extern int unregister_filesystem(struct file_system_type *);
The passed struct file_system_type describes your filesystem. When a request is made to mount a filesystem onto a directory in your namespace, the VFS will call the appropriate mount() method for the specific filesystem. New vfsmount referring to the tree returned by ->mount() will be attached to the mountpoint, so that when pathname resolution reaches the mountpoint it will jump into the root of that vfsmount.
You can see all filesystems that are registered to the kernel in the file /proc/filesystems.
This describes the filesystem. As of kernel 2.6.39, the following members are defined:
struct file_system_operations {
const char *name;
int fs_flags;
struct dentry *(*mount) (struct file_system_type *, int,
const char *, void *);
void (*kill_sb) (struct super_block *);
struct module *owner;
struct file_system_type * next;
struct list_head fs_supers;
struct lock_class_key s_lock_key;
struct lock_class_key s_umount_key;
};
namefs_flagsmountkill_sbownernextfor 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_typeint flagsconst char *dev_namevoid *dataThe 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_bdevmount_nodevmount_singleA fill_super() callback implementation has the following arguments:
struct super_block *sbvoid *dataint silentA superblock object represents a mounted filesystem.
This describes how the VFS can manipulate the superblock of your filesystem. As of kernel 2.6.22, the following members are defined:
struct super_operations {
struct inode *(*alloc_inode)(struct super_block *sb);
void (*destroy_inode)(struct inode *);
void (*dirty_inode) (struct inode *, int flags);
int (*write_inode) (struct inode *, int);
void (*drop_inode) (struct inode *);
void (*delete_inode) (struct inode *);
void (*put_super) (struct super_block *);
int (*sync_fs)(struct super_block *sb, int wait);
int (*freeze_fs) (struct super_block *);
int (*unfreeze_fs) (struct super_block *);
int (*statfs) (struct dentry *, struct kstatfs *);
int (*remount_fs) (struct super_block *, int *, char *);
void (*clear_inode) (struct inode *);
void (*umount_begin) (struct super_block *);
int (*show_options)(struct seq_file *, struct dentry *);
ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
int (*nr_cached_objects)(struct super_block *);
void (*free_cached_objects)(struct super_block *, int);
};
All methods are called without any locks being held, unless otherwise noted. This means that most methods can block safely. All methods are only called from a process context (i.e. not from an interrupt handler or bottom half).
alloc_inodedestroy_inodedirty_inodewrite_inodedrop_inodecalled 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_inodeput_supersync_fsfreeze_fsunfreeze_fsstatfsremount_fsclear_inodeumount_beginshow_optionsquota_readquota_writenr_cached_objectsfree_cache_objectscalled 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.
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.
nameprefixlistgetsetWhen 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.
An inode object represents an object within the filesystem.
This describes how the VFS can manipulate an inode in your filesystem. As of kernel 2.6.22, the following members are defined:
struct inode_operations {
int (*create) (struct inode *,struct dentry *, umode_t, bool);
struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
int (*link) (struct dentry *,struct inode *,struct dentry *);
int (*unlink) (struct inode *,struct dentry *);
int (*symlink) (struct inode *,struct dentry *,const char *);
int (*mkdir) (struct inode *,struct dentry *,umode_t);
int (*rmdir) (struct inode *,struct dentry *);
int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
int (*rename) (struct inode *, struct dentry *,
struct inode *, struct dentry *, unsigned int);
int (*readlink) (struct dentry *, char __user *,int);
const char *(*get_link) (struct dentry *, struct inode *,
struct delayed_call *);
int (*permission) (struct inode *, int);
int (*get_acl)(struct inode *, int);
int (*setattr) (struct dentry *, struct iattr *);
int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
ssize_t (*listxattr) (struct dentry *, char *, size_t);
void (*update_time)(struct inode *, struct timespec *, int);
int (*atomic_open)(struct inode *, struct dentry *, struct file *,
unsigned open_flag, umode_t create_mode);
int (*tmpfile) (struct inode *, struct dentry *, umode_t);
};
Again, all methods are called without any locks being held, unless otherwise noted.
createlookuplinkunlinksymlinkmkdirrmdirmknodrenamecalled 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_linkcalled 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.
readlinkpermissioncalled 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.
setattrgetattrlistxattrupdate_timeatomic_opentmpfileThe 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 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.
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).
This describes how the VFS can manipulate mapping of a file to page cache in your filesystem. The following members are defined:
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*readpage)(struct file *, struct page *);
int (*writepages)(struct address_space *, struct writeback_control *);
int (*set_page_dirty)(struct page *page);
void (*readahead)(struct readahead_control *);
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 *);
};
writepagecalled 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.
readpagewritepagesset_page_dirtyreadaheadreadpageswrite_beginCalled 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_endAfter 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.
bmapinvalidatepagereleasepagereleasepage 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.
freepagedirect_IOisolate_pagemigrate_pageputback_pagelaunder_pageis_partially_uptodateis_dirty_writebackerror_remove_pageswap_activateswap_deactivateA file object represents a file opened by a process. This is also known as an "open file description" in POSIX parlance.
This describes how the VFS can manipulate an open file. As of kernel 4.18, the following members are defined:
struct file_operations {
struct module *owner;
loff_t (*llseek) (struct file *, loff_t, int);
ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
int (*iopoll)(struct kiocb *kiocb, bool spin);
int (*iterate) (struct file *, struct dir_context *);
int (*iterate_shared) (struct file *, struct dir_context *);
__poll_t (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *, fl_owner_t id);
int (*release) (struct inode *, struct file *);
int (*fsync) (struct file *, loff_t, loff_t, int datasync);
int (*fasync) (int, struct file *, int);
int (*lock) (struct file *, int, struct file_lock *);
ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
int (*check_flags)(int);
int (*flock) (struct file *, int, struct file_lock *);
ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
int (*setlease)(struct file *, long, struct file_lock **, void **);
long (*fallocate)(struct file *file, int mode, loff_t offset,
loff_t len);
void (*show_fdinfo)(struct seq_file *m, struct file *f);
#ifndef CONFIG_MMU
unsigned (*mmap_capabilities)(struct file *);
#endif
ssize_t (*copy_file_range)(struct file *, loff_t, struct file *, loff_t, size_t, unsigned int);
loff_t (*remap_file_range)(struct file *file_in, loff_t pos_in,
struct file *file_out, loff_t pos_out,
loff_t len, unsigned int remap_flags);
int (*fadvise)(struct file *, loff_t, loff_t, int);
};
Again, all methods are called without any locks being held, unless otherwise noted.
llseekreadread_iterwritewrite_iteriopolliterateiterate_sharedpollunlocked_ioctlcompat_ioctlmmapopenflushreleasefsyncfasynclockget_unmapped_areacheck_flagsflocksplice_writesplice_readsetleasefallocatecopy_file_rangeremap_file_rangefadviseNote 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.
This describes how a filesystem can overload the standard dentry operations. Dentries and the dcache are the domain of the VFS and the individual filesystem implementations. Device drivers have no business here. These methods may be set to NULL, as they are either optional or the VFS uses a default. As of kernel 2.6.22, the following members are defined:
struct dentry_operations {
int (*d_revalidate)(struct dentry *, unsigned int);
int (*d_weak_revalidate)(struct dentry *, unsigned int);
int (*d_hash)(const struct dentry *, struct qstr *);
int (*d_compare)(const struct dentry *,
unsigned int, const char *, const struct qstr *);
int (*d_delete)(const struct dentry *);
int (*d_init)(struct dentry *);
void (*d_release)(struct dentry *);
void (*d_iput)(struct dentry *, struct inode *);
char *(*d_dname)(struct dentry *, char *, int);
struct vfsmount *(*d_automount)(struct path *);
int (*d_manage)(const struct path *, bool);
struct dentry *(*d_real)(struct dentry *, const struct inode *);
};
d_revalidatecalled 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.
d_weak_revalidatecalled 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_hashcalled 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_comparecalled 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_deleted_initd_released_iputd_dnamecalled when the pathname of a dentry should be generated. Useful for some pseudo filesystems (sockfs, pipefs, ...) to delay pathname generation. (Instead of doing it when dentry is created, it's done only when the path is needed.). Real filesystems probably dont want to use it, because their dentries are present in global dcache hash, so their hash should be an invariant. As no lock is held, d_dname() should not try to modify the dentry itself, unless appropriate SMP safety is used. CAUTION : d_path() logic is quite tricky. The correct way to return for example "Hello" is to put it at the end of the buffer, and returns a pointer to the first char. dynamic_dname() helper function is provided to take care of this.
Example :
static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
{
return dynamic_dname(dentry, buffer, buflen, "pipe:[%lu]",
dentry->d_inode->i_ino);
}
d_automountcalled 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_managecalled 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_realoverlay/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.
There are a number of functions defined which permit a filesystem to manipulate dentries:
dgetdputd_dropd_deleted_addd_instantiated_lookupOn 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.
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.
此处可能存在不合适展示的内容，页面不予展示。您可通过相关编辑功能自查并修改。
如您确认内容无涉及 不当用语 / 纯广告导流 / 暴力 / 低俗色情 / 侵权 / 盗版 / 虚假 / 无价值内容或违法国家有关法律法规的内容，可点击提交进行申诉，我们将尽快为您处理。
马建仓 AI 助手