dentries

Dentries, short for directory entries, are kernel data structures used by the Linux VFS to represent a component of a pathname and to cache its mapping to an inode. They form part of the dcache, a locality-aware cache that speeds up file name lookup and path resolution by keeping recently accessed directory entries in memory. Each dentry corresponds to a single path component (for example, the "docs" in /home/user/docs) and links to its parent dentry, its inode (if the target exists), and any child dentries.

A dentry can be positive, meaning it has an associated inode, or negative, meaning the path component does not exist on disk. Negative dentries cache the absence to avoid repeated filesystem lookups. Dentries are organized as a tree aligned with the directory hierarchy, and the kernel maintains reference counts to manage their lifetimes. When a dentry is no longer referenced, it may be dropped from the dcache, with the inode and other resources released as needed.

Dentries interact with the VFS during name resolution, file creation, deletion, and renaming. They are used to cache lookups so that subsequent operations can skip disk accesses, subject to coherence rules and cache invalidation. Filesystem-specific code provides operations to create or populate dentries, or to invalidate them when the underlying filesystem changes.

Dentries are an internal, kernel-level optimization and are not part of user-space APIs. While they improve performance by reducing I/O, they require careful synchronization and caching strategies to maintain consistency across filesystems and mounted hierarchies.