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diff --git a/pkg/sentry/vfs/README.md b/pkg/sentry/vfs/README.md deleted file mode 100644 index 4b9faf2ea..000000000 --- a/pkg/sentry/vfs/README.md +++ /dev/null @@ -1,195 +0,0 @@ -# The gVisor Virtual Filesystem - -THIS PACKAGE IS CURRENTLY EXPERIMENTAL AND NOT READY OR ENABLED FOR PRODUCTION -USE. For the filesystem implementation currently used by gVisor, see the `fs` -package. - -## 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. - -## Future Work - -- Most `mount(2)` features, and unmounting, are incomplete. - -- VFS1 filesystems are not directly compatible with VFS2. It may be possible - to implement shims that implement `vfs.FilesystemImpl` for - `fs.MountNamespace`, `vfs.DentryImpl` for `fs.Dirent`, and - `vfs.FileDescriptionImpl` for `fs.File`, which may be adequate for - filesystems that are not performance-critical (e.g. sysfs); however, it is - not clear that this will be less effort than simply porting the filesystems - in question. Practically speaking, the following filesystems will probably - need to be ported or made compatible through a shim to evaluate filesystem - performance on realistic workloads: - - - devfs/procfs/sysfs, which will realistically be necessary to execute - most applications. (Note that procfs and sysfs do not support hard - links, so they do not require the complexity of separate inode objects. - Also note that Linux's /dev is actually a variant of tmpfs called - devtmpfs.) - - - tmpfs. This should be relatively straightforward: copy/paste memfs, - store regular file contents in pgalloc-allocated memory instead of - `[]byte`, and add support for file timestamps. (In fact, it probably - makes more sense to convert memfs to tmpfs and not keep the former.) - - - A remote filesystem, either lisafs (if it is ready by the time that - other benchmarking prerequisites are) or v9fs (aka 9P, aka gofers). - - - epoll files. - - Filesystems that will need to be ported before switching to VFS2, but can - probably be skipped for early testing: - - - overlayfs, which is needed for (at least) synthetic mount points. - - - Support for host ttys. - - - timerfd files. - - Filesystems that can be probably dropped: - - - ashmem, which is far too incomplete to use. - - - binder, which is similarly far too incomplete to use. - -- Save/restore. For instance, it is unclear if the current implementation of - the `state` package supports the inheritance pattern described above. - -- Many features that were previously implemented by VFS must now be - implemented by individual filesystems (though, in most cases, this should - consist of calls to hooks or libraries provided by `vfs` or other packages). - This includes, but is not necessarily limited to: - - - Block and character device special files - - - Inotify - - - File locking - - - `O_ASYNC` - -- Reference counts in the `vfs` package do not use the `refs` package since - `refs.AtomicRefCount` adds 64 bytes of overhead to each 8-byte reference - count, resulting in considerable cache bloat. 24 bytes of this overhead is - for weak reference support, which have poor performance and will not be used - by VFS2. The remaining 40 bytes is to store a descriptive string and stack - trace for reference leak checking; we can support reference leak checking - without incurring this space overhead by including the applicable - information directly in finalizers for applicable types. |