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The gVisor Virtual Filesystem

pkg/sentry/vfs/README.md

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The gVisor Virtual Filesystem

Implementation Notes

Reference Counting

Filesystem, Dentry, Mount, MountNamespace, and FileDescription are all reference-counted. Mount and MountNamespace are exclusively VFS-managed; when their reference count reaches zero, VFS releases their resources. Filesystem and FileDescription management is shared between VFS and filesystem implementations; when their reference count reaches zero, VFS notifies the implementation by calling FilesystemImpl.Release() or FileDescriptionImpl.Release() respectively and then releases VFS-owned resources. Dentries are exclusively managed by filesystem implementations; reference count changes are abstracted through DentryImpl, which should release resources when reference count reaches zero.

Filesystem references are held by:

  • Mount: Each referenced Mount holds a reference on the mounted Filesystem.

Dentry references are held by:

  • FileDescription: Each referenced FileDescription holds a reference on the Dentry through which it was opened, via FileDescription.vd.dentry.

  • Mount: Each referenced Mount holds a reference on its mount point and on the mounted filesystem root. The mount point is mutable (mount(MS_MOVE)).

Mount references are held by:

  • FileDescription: Each referenced FileDescription holds a reference on the Mount on which it was opened, via FileDescription.vd.mount.

  • Mount: Each referenced Mount holds a reference on its parent, which is the mount containing its mount point.

  • VirtualFilesystem: A reference is held on each Mount that has been connected to a mount point, but not yet umounted.

MountNamespace and FileDescription references are held by users of VFS. The expectation is that each kernel.Task holds a reference on its corresponding MountNamespace, and each file descriptor holds a reference on its represented FileDescription.

Notes:

  • Dentries do not hold a reference on their owning Filesystem. Instead, all uses of a Dentry occur in the context of a Mount, which holds a reference on the relevant Filesystem (see e.g. the VirtualDentry type). As a corollary, when releasing references on both a Dentry and its corresponding Mount, the Dentry's reference must be released first (because releasing the Mount's reference may release the last reference on the Filesystem, whose state may be required to release the Dentry reference).

The Inheritance Pattern

Filesystem, Dentry, and FileDescription are all concepts featuring both state that must be shared between VFS and filesystem implementations, and operations that are implementation-defined. To facilitate this, each of these three concepts follows the same pattern, shown below for Dentry:

go
// Dentry represents a node in a filesystem tree.
type Dentry struct {
  // VFS-required dentry state.
  parent *Dentry
  // ...

  // impl is the DentryImpl associated with this Dentry. impl is immutable.
  // This should be the last field in Dentry.
  impl DentryImpl
}

// Init must be called before first use of d.
func (d *Dentry) Init(impl DentryImpl) {
  d.impl = impl
}

// Impl returns the DentryImpl associated with d.
func (d *Dentry) Impl() DentryImpl {
  return d.impl
}

// DentryImpl contains implementation-specific details of a Dentry.
// Implementations of DentryImpl should contain their associated Dentry by
// value as their first field.
type DentryImpl interface {
  // VFS-required implementation-defined dentry operations.
  IncRef()
  // ...
}

This construction, which is essentially a type-safe analogue to Linux's container_of pattern, has the following properties:

  • VFS works almost exclusively with pointers to Dentry rather than DentryImpl interface objects, such as in the type of Dentry.parent. This avoids interface method calls (which are somewhat expensive to perform, and defeat inlining and escape analysis), reduces the size of VFS types (since an interface object is two pointers in size), and allows pointers to be loaded and stored atomically using sync/atomic. Implementation-defined behavior is accessed via Dentry.impl when required.

  • Filesystem implementations can access the implementation-defined state associated with objects of VFS types by type-asserting or type-switching (e.g. Dentry.Impl().(*myDentry)). Type assertions to a concrete type require only an equality comparison of the interface object's type pointer to a static constant, and are consequently very fast.

  • Filesystem implementations can access the VFS state associated with objects of implementation-defined types directly.

  • VFS and implementation-defined state for a given type occupy the same object, minimizing memory allocations and maximizing memory locality. impl is the last field in Dentry, and Dentry is the first field in DentryImpl implementations, for similar reasons: this tends to cause fetching of the Dentry.impl interface object to also fetch DentryImpl fields, either because they are in the same cache line or via next-line prefetching.