[TASK] Phase 4.2.2: In-Memory Filesystem (tmpfs)

[iamvirul]

Overview

Implement tmpfs: a simple, heap-backed in-memory filesystem that mounts at /. This is the first concrete Filesystem implementation and validates the VFS layer. All data is lost on reboot — that is expected and correct.

Design

Storage Model

pub struct TmpfsInode {
    id: InodeId,
    kind: InodeKind,
    data: SpinLock<TmpfsData>,
}

pub enum TmpfsData {
    File { content: Vec<u8> },
    Dir  { entries: BTreeMap<String, Arc<TmpfsInode>> },
    Symlink { target: String },
}

Inode impl

• lookup(name) -> BTreeMap lookup in Dir
• open(flags) -> returns TmpfsFile wrapping a clone of the Arc<TmpfsInode>
• create(name, kind) -> insert new TmpfsInode into parent Dir
• unlink(name) -> remove from parent Dir; drop Arc (freed when last reference gone)
• readdir() -> iterate BTreeMap keys

TmpfsFile read/write

• read(buf, offset) -> copy content[offset..offset+buf.len()]
• write(buf, offset) -> extend or overwrite content; update size

Acceptance Criteria

• tmpfs mounts at / during kernel init
• create / lookup / unlink work for files and directories
• read / write work for files (arbitrary offset, arbitrary size)
• readdir returns correct entries
• Files persist across multiple open calls within the same boot
• Smoke test: create /hello.txt, write "Hello", open again, read "Hello"
• Smoke test: create /dir/, create /dir/file.txt, readdir("/dir/") returns ["file.txt"]
• Serial log: "tmpfs: mounted at /", "tmpfs: created /hello.txt", "tmpfs: read 5 bytes from /hello.txt"

Files

• kernel/src/fs/tmpfs/mod.rs
• kernel/src/fs/tmpfs/inode.rs
• kernel/src/fs/tmpfs/file.rs

Dependencies

• Phase 4.2.1 (VFS layer): Filesystem + Inode + File traits