Following a File Through the Linux 2.4 VFS

Tue, 12 Nov 2002 14:56:03 +0000

I was reading a small filesystem and kept confusing dentries, inodes, and open files. They all seemed to point at roughly the same object until a rename and two simultaneous opens proved otherwise. I finally followed one pathname through the Linux 2.4 VFS and wrote down what each structure represents.

The VFS is the layer that gives user programs the usual operations such as open, read, and stat while allowing ext2, NFS, procfs, and other filesystems to implement different storage rules. It does this with common objects and tables of function pointers. The useful mental model is not one structure per file. It is several structures per role.

A filesystem implementation first registers a file_system_type:

static DECLARE_FSTYPE(sample_type, "samplefs", sample_read_super,
 FS_REQUIRES_DEV);

static int __init sample_init(void)
{
 return register_filesystem(&sample_type);
}

In 2.4, mounting leads to the filesystem’s read_super function. It validates or reads the on-disk superblock, fills the VFS super_block, installs superblock operations, and supplies a root dentry. The superblock represents one mounted filesystem instance, not the filesystem format in general. Two mounted ext2 partitions therefore have separate superblocks while sharing ext2’s implementation code.

The inode represents a filesystem object and its metadata: type, mode, owner, times, size, and operation tables. A filesystem usually embeds or associates its private inode information with the VFS inode. The inode number identifies it within that filesystem. Hard links make the distinction important: several directory names can refer to the same inode.

The dentry represents a name in a directory and connects pathname lookup to an inode. It belongs to the dentry cache, remembers parent and child relationships, and may even be negative, meaning that a lookup established that a name does not currently exist. A dentry is therefore about a pathname component and cached lookup state, not merely another spelling of inode.

Suppose a process opens /mnt/notes/todo. Pathname lookup starts from a root or current directory and walks components. For each component, the VFS uses cached dentries when possible and asks the parent inode’s lookup operation when necessary. By the end, it has a dentry for todo and, if the file exists, an associated inode.

Opening then creates a file object for that particular open instance. It records the current offset, access mode, flags, and file operations. Two processes opening the same inode receive distinct file objects and can have different offsets. After fork, file descriptors may instead refer to the same open file object, which explains why their offsets can move together.

The operation tables divide responsibility. Directory inode operations include tasks such as lookup, create, link, and rename. File operations include read, write, llseek, open, and release. Superblock operations deal with the mounted instance, including writing metadata and releasing it. The VFS calls through these tables without needing ext2 rules in generic pathname code.

A tiny read method illustrates another boundary:

static ssize_t sample_read(struct file *file, char *buf,
 size_t count, loff_t *ppos)
{
 const char text[] = "hello\n";
 size_t left;

 if (*ppos >= sizeof(text) - 1)
 return 0;

 left = sizeof(text) - 1 - *ppos;
 if (count > left)
 count = left;
 if (copy_to_user(buf, text + *ppos, count))
 return -EFAULT;

 *ppos += count;
 return count;
}

The method receives the open file, not just the inode, because position and open flags belong to the open instance. It uses copy_to_user() because a user pointer cannot be treated as an ordinary kernel pointer. Returning zero at end of file is part of the read contract, not an error.

Caching makes lifetime rules less obvious. Closing the last file descriptor does not imply that the inode or dentry disappears immediately; the caches may retain them. Conversely, unlinking a name removes a directory entry but an already open file can remain usable until its final reference is released. Reference counts and VFS helper functions, rather than guesses about user-visible names, govern these lifetimes.

Locking also follows roles. Operations changing a directory need the directory inode’s semaphore according to the VFS calling rules, while filesystem-private data may need its own protection. I do not add locks at random around every pointer. I first determine which VFS locks are already held for that operation and what shared state remains uncovered. Too little locking corrupts data; too much can deadlock, which is corruption with better manners.

My preferred debugging method is to trace one operation and print object addresses and names carefully: mount, lookup, open, read, release, and unmount. A toy in-memory filesystem is especially educational because there is no disk format to distract from the object relationships.

The concise version is: a superblock is a mount, an inode is an object, a dentry is a cached name, and a file is an open instance. It is not the whole VFS, but it is enough to stop asking an inode for an open offset. That alone made the source considerably less mysterious.

Vfs on Roberto Selbach

Following a File Through the Linux 2.4 VFS