DentryCache

DentryCache, commonly referred to as the dcache, is a kernel-level cache used by the Linux virtual file system (VFS) to store directory entry objects (dentries). It maps path components to their corresponding inodes and speeds up path name resolution by avoiding repeated directory reads and inode lookups.

A dentry represents a single component of a file path and can be positive (linking to a specific inode) or negative (the name does not exist in the given directory). Dentries are typically shared across processes and mount namespaces, and they are organized to enable fast lookup, often via a hash-based structure keyed by the parent dentry and the component name. Each dentry carries references to its inode when valid and may exist without an inode reference in the negative case.

Lifecycle and operations are oriented around the path resolution process. When a path component is accessed, the kernel consults the dcache to find or create the corresponding dentry. Functions and mechanisms exist to insert, look up, move, or drop dentries (for example, during renames, unlink operations, or mount changes). Dentries are reference-counted to manage their lifetime and ensure proper reuse or release as processes traverse or terminate paths.

Consistency and invalidation are maintained through the dcache’s interaction with the underlying filesystem. Dentries can become stale if the corresponding inode changes, files are renamed or deleted, or a filesystem is unmounted. Invalidation may involve revalidation checks or dropping outdated dentries, including negative entries that previously indicated nonexistence.

Performance considerations include the trade-off between faster lookups and the risk of stale path information. Under memory pressure, the dcache is shrunk to reclaim memory, evicting dentries that are not actively referenced.