# The VFS The layer between `open(2)` and a filesystem. SchFS is described separately in [schfs.md](schfs.md); this is the machinery it plugs into, plus the buffer cache, mounts, and procfs. --- ## `struct fs_ops` A filesystem is a vtable, chosen by `iget()` from the device number and never null once an inode is held. ```c struct fs_ops { const char *name; void (*iload) (struct inode *); // fill type/size/nlink from disk void (*istore) (struct inode *); // write them back void (*ifree) (struct inode *); // last link gone, release it int (*read) (struct inode *, int user_dst, uint64 dst, uint64 off, uint n); int (*write) (struct inode *, int user_src, uint64 src, uint64 off, uint n); void (*truncate)(struct inode *, uint64 newsize); struct inode *(*dirlookup)(struct inode *dp, char *name, uint *poff); int (*dirlink) (struct inode *dp, char *name, uint64 inum); int (*dirunlink)(struct inode *dp, char *name, uint off); int (*dirempty) (struct inode *dp); struct inode *(*ialloc)(uint dev, short type); int (*getdents)(struct inode *dp, char *out, int cap, uint64 *off); }; ``` Two implementations exist: `schfs_ops` (`kernel/fs/schfs/vfs.c`) and `procfs_ops` (`kernel/fs/procfs.c`). `poff` is deliberately **an opaque cookie** rather than a byte offset — meaningful only to the same filesystem's `dirunlink`. A directory does not have to be an array of fixed-size records for this interface to work. `ialloc` creates an object **with no name**. The caller sets `nlink` and calls `dirlink`; until it does, nothing can reach the object. If the caller dies in between, the result is an unreferenced record that is harmless and, in SchFS, never reaches the disk anyway — nothing is durable until the transaction commits. ### The one place the models disagree The VFS inherited xv6's shape: allocate an inode, fill it in, link a name to it. SchFS has no free-standing inode — an object exists because a record with its id exists. `nlink` is the sharper disagreement. **The VFS adjusts it itself** and then calls `iupdate`, so SchFS's own link accounting is deliberately bypassed in the VFS path: `schfs_dir_add`/`schfs_dir_del` are used rather than `schfs_link`/ `schfs_unlink`, and `iput`'s `ifree` hook releases the object once the VFS has decided there are no names left. > Two pieces of code counting links independently is precisely how a filesystem > loses a file that is still referenced. `nlink` is an `int`, not a `short`. SchFS stores a `uint32` and the narrower type truncated it, so a file past 32767 names came back with a *negative* count — and `iput` frees an object whose count reaches zero. Signed on purpose: `nlink--` on a count that is somehow already zero should land somewhere a check can see it, not wrap to four billion. ## Path resolution `namex()` walks a path one component at a time. It was refactored into `namex_from(struct inode *start, ...)`, parameterising what used to be a hardcoded `idup(proc->cwd)`. That refactor is what made the `*at()` family work. Every one of them — `openat`, `fstatat`, `unlinkat`, `renameat`, `linkat`, `symlinkat`, `mkdirat`, `mknodat`, `readlinkat`, `faccessat` — previously handled only two cases: `AT_FDCWD`, or an absolute path with `dirfd` ignored. A **real** dirfd with a **relative** path is the standard, efficient way any directory-tree walker avoids rebuilding full path strings, and it was either rejected outright or — worse, in `openat` — silently resolved against `proc->cwd`, which can open the wrong file entirely. `sys_fs.c` now has `at_dir(dirfd)`, `at_namei()` and `at_nameiparent()`, and every `*at()` call uses them. ## The buffer cache `kernel/dev/bio.c`, xv6's design unchanged: a fixed pool of buffers, an LRU list, one sleeplock per buffer. `bread()` returns a locked buffer; `bwrite()` flushes it; `brelse()` releases. **Sleeping is the point.** A buffer lock is held across a disk read, which is why every path that touches a filesystem must be able to sleep — and why the root mount happens in `forkret()` rather than in `main()`. ## Mounts `mount()` and `umount2()` at their Linux numbers, with a mount table consulted during path resolution. `/proc` is mounted by init: ```c mount("proc", "/proc", "proc", 0, 0); ``` ## procfs `kernel/fs/procfs.c`. A real filesystem behind the same vtable, not a set of character devices. **The reason it had to become one is `ps`.** /proc used to be `mknod`'d nodes — init ran `mknod("/proc/uptime", 6, 2)` and friends, so every file was a real inode on the root filesystem with a major/minor behind it. That is fine for a fixed list and cannot work at all for the thing /proc is mostly for: **one directory per process**. You cannot `mknod` a directory for a process that does not exist yet and will be gone in a minute. `ps` listed `/proc`, got no numbers, and reported nothing. It was never a `ps` problem. Nothing is stored. Directory listings are generated from the process table when asked; file contents when read. Three things worth knowing: - **The split is deliberate.** `kernel/fs/procfs.c` owns the *namespace* — what exists. `kernel/dev/procfs.c` keeps the *generators* — what the global files say. They change for different reasons, and `proc_gen_global()` is the only thing that crosses between them. - **procfs never walks the process table.** `proc_snapshot()` copies what is needed out from under `ptable.lock`, so a process that exits between the readdir that listed it and the open that reads it produces a stale line rather than a use-after-free. - **Sizes are reported as zero**, exactly as Linux does. A synthetic file has no length until it is generated, and generating every one on every `stat()` to fill in a number almost nothing reads would be absurd. ### What is there Per-process directories, plus the global files: `meminfo`, `uptime`, `cpuinfo`, `version`, `stat`, `loadavg`, `mounts`, `filesystems`, `schfs`, `pci`, `interrupts`, `devices`, `netif`, and `sysrq`. **/proc is read-only except `sysrq`**, which is the one deliberate write target, and the code says so in one place. Marking every procfs inode read-only once broke `echo s > /proc/sysrq` and took out three USB checks within a minute — they drive the disk through sysrq. `sysrq` keys: `c` crash on purpose, `u` / `s` storage checks, `d` display selftest, `t` text specimen, `n` the netdev ARP selftest. --- ## What it does not do - **No page cache.** `mmap` of a file reads the whole thing in at map time, and writable `MAP_SHARED` of a file is refused rather than faked. This is the single largest gap in the layer. - **No symlinks in procfs**, which is why `/proc//exe` is a regular file and `ps -o comm` prints `[init]`. - **No `AF_UNIX` socket inodes.** Socket names live in a table in `kernel/net/unix.c`, so `bind()` creates nothing that can be `stat()`ed or `unlink()`ed. - **No unified inode cache across filesystems** beyond the single `itable`. - **A known, un-root-caused hang:** heavy `ls -lR` use across many commands in one long shell session can eventually make a *later*, unrelated directory hang; the identical command succeeds on a fresh boot. It is not inode or file table exhaustion — both fail cleanly and neither fired. It smells like a lock or refcount leak in the `*at()` paths or in toybox's own fd handling. The next step is instrumenting `itable`/`ftable` occupancy across a long session to see what is climbing. - **`sys_delete`/`sys_show`/`sys_showstat`** are orphaned entries from the old v1 filesystem's recycle bin. Nothing reaches them and they die with `fs.c`.