Kernel: vnode Operations

Summary of known vfs operations and vnode operations.

1. VFS Operations (vfsops_t)

These are operations on the filesystem as a whole:

Operation	Purpose / Semantics	Typical FS Relevance
mount	Mount the filesystem; initialize structures, link root vnode.	All FS
umount	Unmount; flush buffers, release resources.	All FS
sync	Flush all dirty buffers and VM pages to disk.	Persistent FS (EFS, XFS)
statvfs	Retrieve filesystem statistics (free space, block size).	All FS
vget	Retrieve vnode by inode number or identifier.	Persistent FS
quotactl	Manage quotas on filesystem (user/group limits).	Optional, persistent FS
import	Integrate remote filesystem structures (NFS, exported FS).	Network FS
root	Return root vnode of filesystem.	All FS
reclaim	Cleanup filesystem-specific vnode data before freeing vnode memory.	All FS
fsid	Return filesystem ID.	Persistent FS
check	Perform consistency or integrity checks (optional).	Persistent FS
remount	Update mount options without unmounting.	Optional
statfs	Return summarized filesystem status.	Persistent FS

Notes:

• Pseudo-filesystems (pipefs, fifofs) stub most of these functions; only root or vget may return in-memory structures.
• Real filesystems (EFS, XFS) implement almost all operations for proper filesystem management.

---

2. Vnode Operations (vnodeops_t)

These are operations on individual vnodes, dispatched via the BHV layer.

Operation	Purpose / Semantics	FS Relevance / Typical Implementation
read	Read data from vnode into user or kernel buffer.	All FS supporting files or streams
write	Write data to vnode from buffer.	All FS supporting files or streams
ioctl	Perform device-specific or FS-specific commands.	Devices, pseudo-FS, streams
getattr	Retrieve vnode attributes (mode, owner, size, timestamps).	All FS
setattr	Update vnode attributes.	All FS
access	Check access permissions.	All FS
lookup	Resolve pathname component to vnode.	Directory FS (EFS, XFS, pipefs limited)
create	Create a new file.	Persistent FS; pseudo-FS may stub
remove	Remove a file.	Persistent FS; pipefs/FIFO may implement
mkdir	Create directory.	Persistent FS
rmdir	Remove directory.	Persistent FS
readdir	Read directory entries.	Persistent FS
symlink	Create symbolic link.	Optional, persistent FS
readlink	Read symbolic link target.	Optional, persistent FS
rename	Rename a file.	Persistent FS
link	Create hard link.	Persistent FS
unlink	Remove a link.	Persistent FS
fsync	Flush vnode’s buffers to disk.	Persistent FS
map	Map file pages into VM.	Persistent FS, memory-mapped files
addmap	Notify FS of memory mapping.	Persistent FS
delmap	Notify FS of unmapped region.	Persistent FS
frlock	File region locking.	All FS supporting advisory locks
poll	Determine readiness for I/O.	Streams, pipes, devices
seek	Validate seek operation.	Regular files, optionally pseudo-FS
inactive	Called when last reference released; free resources.	All FS
reclaim	Called to cleanup filesystem-specific vnode state.	All FS
realvp	Return underlying “real” vnode in layered FS.	Stacked or layered FS
cover	Return vnode that this vnode overlays.	Stacked FS
link_removed	Notify FS that a link was removed.	Persistent FS

Notes:

• Pseudo-filesystems (pipefs, fifofs):
  • Implement read, write, poll, inactive
  • Many other operations are stubs (fs_nosys) because they have no persistent storage.
• Persistent filesystems (EFS, XFS):
  • Implement almost all operations fully.
  • VM integration (map, addmap, delmap) links vnode buffers with page cache.
  • Locking (frlock) and attribute operations are critical for filesystem consistency.
• Streams / devices:
  • Integrate with poll / select via pollhead structures.
  • ioctl is heavily used to expose device-specific functionality.

---

3. Behavior Semantics

• Vnode operations are dispatched via the behavior chain, allowing:
  • Layered filesystems: stacking additional behaviors (e.g., shadow vnodes, network FS).
  • Pseudo-filesystems: handle vnodes without disk backing.
• Operations are always invoked on BHV descriptors, not directly on vnodes. This ensures correct dispatch in multi-layer environments.

---

4. Filesystem Type Considerations

FS Type	Implemented Operations
Persistent (EFS/XFS)	All VFS and vnode ops; full VM/page cache integration, locking, attributes
Pseudo-FS (pipefs/fifofs)	read, write, poll, inactive; stubs for most others
Device FS (devfs)	read, write, ioctl, poll, getattr/setattr
Network FS (NFS-like)	import, vget, root, lookup, read, write

---

5. Reference / Lifecycle Management

• VFS ops: reclaim and sync ensure global cleanup.
• Vnode ops: inactive and reclaim ensure per-vnode cleanup.
• BHVs: behavior chain allows multiple layers to participate in cleanup or dispatch.