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fs.c
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fs.c
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// File system implementation. Five layers:
// + Blocks: allocator for raw disk blocks.
// + Log: crash recovery for multi-step updates.
// + Files: inode allocator, reading, writing, metadata.
// + Directories: inode with special contents (list of other inodes!)
// + Names: paths like /usr/rtm/xv6/fs.c for convenient naming.
//
// This file contains the low-level file system manipulation
// routines. The (higher-level) system call implementations
// are in sysfile.c.
#include "types.h"
#include "defs.h"
#include "param.h"
#include "stat.h"
#include "mmu.h"
#include "proc.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "buf.h"
#include "file.h"
#define min(a, b) ((a) < (b) ? (a) : (b))
static void itrunc(struct inode*);
// there should be one superblock per disk device, but we run with
// only one device
struct superblock sb;
// Read the super block.
void
readsb(int dev, struct superblock *sb)
{
struct buf *bp;
bp = bread(dev, 1);
memmove(sb, bp->data, sizeof(*sb));
brelse(bp);
}
// Zero a block.
static void
bzero(int dev, int bno)
{
struct buf *bp;
bp = bread(dev, bno);
memset(bp->data, 0, BSIZE);
log_write(bp);
brelse(bp);
}
// Blocks.
// Allocate a zeroed disk block and return it, unlocked.
static uint
balloc(uint dev)
{
int b, bi, m;
struct buf *bp;
bp = 0;
for(b = 0; b < sb.size; b += BPB){
bp = bread(dev, BBLOCK(b, sb));
for(bi = 0; bi < BPB && b + bi < sb.size; bi++){
m = 1 << (bi % 8);
if((bp->data[bi/8] & m) == 0){ // Is block free?
bp->data[bi/8] |= m; // Mark block in use.
log_write(bp);
brelse(bp);
bzero(dev, b + bi);
return b + bi;
}
}
brelse(bp);
}
panic("balloc: out of blocks");
}
// Free a disk block.
static void
bfree(int dev, uint b)
{
struct buf *bp;
int bi, m;
readsb(dev, &sb);
bp = bread(dev, BBLOCK(b, sb));
bi = b % BPB;
m = 1 << (bi % 8);
if((bp->data[bi/8] & m) == 0)
panic("freeing free block");
bp->data[bi/8] &= ~m;
log_write(bp);
brelse(bp);
}
// Inodes.
//
// An inode describes a single unnamed file.
// The inode disk structure holds metadata: the file's type,
// its size, the number of links referring to it, and the
// list of blocks holding the file's content.
//
// The inodes are laid out sequentially on disk at
// sb.startinode. Each inode has a number, indicating its
// position on the disk.
//
// The kernel keeps a cache of in-use inodes in memory
// to provide a place for synchronizing access
// to inodes used by multiple processes. The cached
// inodes include book-keeping information that is
// not stored on disk: ip->ref and ip->flags.
//
// All uses of struct ip* refer to an entry in this inode cache.
//
// An inode and its in-memory representative go through a
// sequence of states before they can be used by the
// rest of the file system code.
//
// * Allocation: an inode is allocated if its type (on disk)
// is non-zero. ialloc() allocates, iput() frees if
// the link count has fallen to zero.
//
// * Referencing in cache: an entry in the inode cache
// is free if ip->ref is zero. Otherwise ip->ref tracks
// the number of in-memory pointers to the entry (open
// files and current directories). iget() to find or
// create a cache entry and increment its ref, iput()
// to decrement ref.
//
// * Valid: the information (type, size, &c) in an inode
// cache entry is only correct when the I_VALID bit
// is set in ip->flags. ilock() reads the inode from
// the disk and sets I_VALID, while iput() clears
// I_VALID if ip->ref has fallen to zero.
//
// * Locked: file system code may only examine and modify
// the information in an inode and its content if it
// has first locked the inode.
//
// Thus a typical sequence is:
// ip = iget(dev, inum) // get icache reference
// ilock(ip) // load data from disk
// ... examine and modify ip->xxx ...
// iunlock(ip)
// iput(ip)
//
// ilock() is separate from iget() so that system calls can
// get a long-term reference to an inode (as for an open file)
// and only lock it for short periods (e.g., in read()).
// The separation also helps avoid deadlock and races during
// pathname lookup. iget() increments ip->ref so that the inode
// stays cached and pointers to it remain valid.
//
// Many internal file system functions expect the caller to
// have locked the inodes involved; this lets callers create
// multi-step atomic operations.
struct {
struct spinlock lock;
struct inode inode[NINODE];
} icache;
void
iinit(int dev)
{
int i = 0;
initlock(&icache.lock, "icache");
for(i = 0; i < NINODE; i++) {
initsleeplock(&icache.inode[i].lock, "inode");
}
readsb(dev, &sb);
cprintf("sb: size %d nblocks %d ninodes %d nlog %d logstart %d\
inodestart %d bmap start %d\n", sb.size, sb.nblocks,
sb.ninodes, sb.nlog, sb.logstart, sb.inodestart,
sb.bmapstart);
}
static struct inode* iget(uint dev, uint inum);
//PAGEBREAK!
// Allocate a new inode with the given type on device dev.
// A free inode has a type of zero.
struct inode*
ialloc(uint dev, short type)
{
int inum;
struct buf *bp;
struct dinode *dip;
for(inum = 1; inum < sb.ninodes; inum++){
bp = bread(dev, IBLOCK(inum, sb));
dip = (struct dinode*)bp->data + inum%IPB;
if(dip->type == 0){ // a free inode
memset(dip, 0, sizeof(*dip));
dip->type = type;
log_write(bp); // mark it allocated and dirty in the buffer cache
brelse(bp);
return iget(dev, inum);
}
brelse(bp);
}
panic("ialloc: no inodes");
}
// Copy a modified in-memory inode to disk via the log.
void
iupdate(struct inode *ip)
{
struct buf *bp;
struct dinode *dip;
// Get the on-disk inode contents into the
// buffer cache
bp = bread(ip->dev, IBLOCK(ip->inum, sb));
dip = (struct dinode*)bp->data + ip->inum%IPB;
dip->type = ip->type;
dip->major = ip->major;
dip->minor = ip->minor;
dip->nlink = ip->nlink;
dip->size = ip->size;
memmove(dip->addrs, ip->addrs, sizeof(ip->addrs));
log_write(bp);
brelse(bp);
}
// Get the icache entry for an inode, or an empty
// cache entry if it's not in there, containing the
// desired inum and ready to be populated with `ilock`.
// Does not lock the inode and does not read it from disk,
// to avoid deadlocks when using dirlookup, and to allow
// code to have long-running, non-exclusive references
// to inodes.
static struct inode*
iget(uint dev, uint inum)
{
struct inode *ip, *empty;
acquire(&icache.lock);
// Is the inode already cached?
empty = 0;
for(ip = &icache.inode[0]; ip < &icache.inode[NINODE]; ip++){
if(ip->ref > 0 && ip->dev == dev && ip->inum == inum){
ip->ref++;
release(&icache.lock);
return ip;
}
if(empty == 0 && ip->ref == 0) // Remember empty slot.
empty = ip;
}
// Recycle an inode cache entry.
if(empty == 0)
panic("iget: no inodes");
ip = empty;
ip->dev = dev;
ip->inum = inum;
ip->ref = 1;
ip->flags = 0;
release(&icache.lock);
return ip;
}
// Increment reference count for ip.
// Returns ip to enable ip = idup(ip1) idiom.
struct inode*
idup(struct inode *ip)
{
acquire(&icache.lock);
ip->ref++;
release(&icache.lock);
return ip;
}
// Lock the given inode.
// Reads the inode from disk if necessary.
void
ilock(struct inode *ip)
{
struct buf *bp;
struct dinode *dip;
if(ip == 0 || ip->ref < 1)
panic("ilock");
acquiresleep(&ip->lock);
if(!(ip->flags & I_VALID)){
bp = bread(ip->dev, IBLOCK(ip->inum, sb));
dip = (struct dinode*)bp->data + ip->inum%IPB;
ip->type = dip->type;
ip->major = dip->major;
ip->minor = dip->minor;
ip->nlink = dip->nlink;
ip->size = dip->size;
memmove(ip->addrs, dip->addrs, sizeof(ip->addrs));
brelse(bp);
ip->flags |= I_VALID;
if(ip->type == 0)
panic("ilock: no type");
}
}
// Unlock the given inode.
void
iunlock(struct inode *ip)
{
if(ip == 0 || !holdingsleep(&ip->lock) || ip->ref < 1)
panic("iunlock");
releasesleep(&ip->lock);
}
// Drop a reference to an in-memory inode.
// If that was the last reference, the inode cache entry can
// be recycled.
// If that was the last reference and the inode has no links
// to it, free the inode (and its content) on disk.
// All calls to iput() must be inside a transaction in
// case it has to free the inode.
void
iput(struct inode *ip)
{
acquire(&icache.lock);
if(ip->ref == 1 && (ip->flags & I_VALID) && ip->nlink == 0){
// inode has no links and no other references: truncate and free.
release(&icache.lock);
itrunc(ip);
ip->type = 0; // Why don't we just do this in itrunc?
iupdate(ip); // ...and avoid the need for this iupdate call?
acquire(&icache.lock);
ip->flags = 0;
}
ip->ref--;
release(&icache.lock);
}
// Common idiom: unlock, then put.
void
iunlockput(struct inode *ip)
{
iunlock(ip);
iput(ip);
}
//PAGEBREAK!
// Inode content
//
// The content (data) associated with each inode is stored
// in blocks on the disk. The first NDIRECT block numbers
// are listed in ip->addrs[]. The next NINDIRECT blocks are
// listed in block ip->addrs[NDIRECT].
// Return the disk block address of the nth logical block in inode ip.
// If there is no such block, bmap allocates one. Since bmap eagerly
// allocates like this, it's important for callers to be careful with bn
// if they don't want unexpected block allocations. See `readi` below.
// Note that bmap doesn't call iupdate--the caller must detect if
// bmap allocated blocks and then call it themselves (see writei below).
static uint
bmap(struct inode *ip, uint bn)
{
uint addr, *a;
struct buf *bp;
// Direct block
if(bn < NDIRECT){
if((addr = ip->addrs[bn]) == 0)
// Logical block not yet allocated
ip->addrs[bn] = addr = balloc(ip->dev);
return addr;
}
bn -= NDIRECT;
// Indirect block
if(bn < NINDIRECT){
if((addr = ip->addrs[NDIRECT]) == 0)
// Indirect block itself not yet allocated
ip->addrs[NDIRECT] = addr = balloc(ip->dev);
// Read+lock indirect block
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn]) == 0){
// Logical block not yet allocated
a[bn] = addr = balloc(ip->dev);
log_write(bp);
}
brelse(bp);
return addr;
}
bn -= NINDIRECT;
// Double-indirect block
if (bn < NINDIRECT*NINDIRECT) {
if((addr = ip->addrs[NDIRECT + 1]) == 0)
// First double-indirect block not yet allocated
ip->addrs[NDIRECT + 1] = addr = balloc(ip->dev);
// Read+lock first double-indirect block
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn / NINDIRECT]) == 0) {
// Second double-indirect block not yet allocated
a[bn / NINDIRECT] = addr = balloc(ip->dev);
log_write(bp);
}
brelse(bp);
// Read+lock second double-indirect block
bp = bread(ip->dev, addr);
a = (uint*)bp->data;
if((addr = a[bn % NINDIRECT]) == 0) {
// Logical block not yet allocated
a[bn % NINDIRECT] = addr = balloc(ip->dev);
log_write(bp);
}
brelse(bp);
return addr;
}
panic("bmap: out of range");
}
// Truncate inode (discard contents).
// Only called when the inode has no links
// to it (no directory entries referring to it)
// and has no in-memory reference to it (is
// not an open file or current directory).
static void
itrunc(struct inode *ip)
{
int i, j;
struct buf *bp;
uint *a;
// Free direct blocks
for(i = 0; i < NDIRECT; i++){
if(ip->addrs[i]){
bfree(ip->dev, ip->addrs[i]);
ip->addrs[i] = 0;
}
}
// Free indirect blocks
if(ip->addrs[NDIRECT]){
bp = bread(ip->dev, ip->addrs[NDIRECT]);
a = (uint*)bp->data;
for(j = 0; j < NINDIRECT; j++){
if(a[j])
bfree(ip->dev, a[j]);
}
brelse(bp);
bfree(ip->dev, ip->addrs[NDIRECT]); // Free the indirect block itself
ip->addrs[NDIRECT] = 0;
}
ip->size = 0;
iupdate(ip);
}
// Copy stat information from inode.
void
stati(struct inode *ip, struct stat *st)
{
st->dev = ip->dev;
st->ino = ip->inum;
st->type = ip->type;
st->nlink = ip->nlink;
st->size = ip->size;
}
//PAGEBREAK!
// Read data from a file into *dst via the file's
// inode.
int
readi(struct inode *ip, char *dst, uint off, uint n)
{
uint tot, m;
struct buf *bp;
if(ip->type == T_DEV){
if(ip->major < 0 || ip->major >= NDEV || !devsw[ip->major].read)
return -1;
return devsw[ip->major].read(ip, dst, n);
}
if(off > ip->size || off + n < off)
return -1;
if(off + n > ip->size)
// User tried to read past end of file.
// This would cause `bmap` to allocate blocks
// to accomodate. We don't want that.
n = ip->size - off;
for(tot=0; tot<n; tot+=m, off+=m, dst+=m){
bp = bread(ip->dev, bmap(ip, off/BSIZE));
m = min(n - tot, BSIZE - off%BSIZE);
/*
cprintf("data off %d:\n", off);
for (int j = 0; j < min(m, 10); j++) {
cprintf("%x ", bp->data[off%BSIZE+j]);
}
cprintf("\n");
*/
memmove(dst, bp->data + off%BSIZE, m);
brelse(bp);
}
return n;
}
// PAGEBREAK!
// Write to a file via the transaction log, through the
// the file's inode.
int
writei(struct inode *ip, char *src, uint off, uint n)
{
uint tot, m;
struct buf *bp;
if(ip->type == T_DEV){
if(ip->major < 0 || ip->major >= NDEV || !devsw[ip->major].write)
return -1;
return devsw[ip->major].write(ip, src, n);
}
if(off > ip->size || off + n < off)
return -1;
if(off + n > MAXFILE*BSIZE)
return -1;
for(tot=0; tot<n; tot+=m, off+=m, src+=m){
bp = bread(ip->dev, bmap(ip, off/BSIZE)); // Round offset down to nearest block
// m is the smallest of 1) num bytes
// to n and 2) num bytes to next disk block.
// Thus, in each iteration, we're copying
// data in a block-aligned fashion until the
// last iteration, which aligns with n.
m = min(n - tot, BSIZE - off%BSIZE);
memmove(bp->data + off%BSIZE, src, m);
log_write(bp);
brelse(bp);
}
if(n > 0 && off > ip->size){
// File grew--update metadata
ip->size = off;
iupdate(ip);
}
return n;
}
//PAGEBREAK!
// Directories
int
namecmp(const char *s, const char *t)
{
return strncmp(s, t, DIRSIZ);
}
// Look for a directory entry in a directory.
// If found, set *poff to byte offset of entry
// within the parent directory, and return the unlocked
// inode corresponding to the found directory entry.
// Otherwise return 0.
// This function is the reason why iget returns
// an unlocked icache reference instead of a locked
// and populated inode. The caller of this function has
// locked `dp`--if they were searching for '.', we'd
// try to re-acquire a held lock, resulting in deadlock.
// Other more complicated situations are possible involving
// '..'. See `namex`.
// If caller wants to read/write the target inode, they
// should unlock dp and then lock the new inode.
struct inode*
dirlookup(struct inode *dp, char *name, uint *poff)
{
uint off;
struct dirent de;
if(dp->type != T_DIR)
panic("dirlookup not DIR");
// Read each dirent and match on the name string
for(off = 0; off < dp->size; off += sizeof(de)){
if(readi(dp, (char*)&de, off, sizeof(de)) != sizeof(de))
panic("dirlookup read");
if(de.inum == 0)
continue;
if(namecmp(name, de.name) == 0){
// entry matches path element
if(poff)
*poff = off;
return iget(dp->dev, de.inum);
}
}
return 0;
}
// Write a new directory entry (name, inum) into the directory dp,
// growing dp if necessary. This is a "hard link".
int
dirlink(struct inode *dp, char *name, uint inum)
{
int off;
struct dirent de;
struct inode *ip;
// Check that name is not present.
if((ip = dirlookup(dp, name, 0)) != 0){
iput(ip);
return -1;
}
// Look for an empty dirent.
for(off = 0; off < dp->size; off += sizeof(de)){
if(readi(dp, (char*)&de, off, sizeof(de)) != sizeof(de))
panic("dirlink read");
if(de.inum == 0)
break;
}
strncpy(de.name, name, DIRSIZ);
de.inum = inum;
if(writei(dp, (char*)&de, off, sizeof(de)) != sizeof(de))
panic("dirlink");
return 0;
}
//PAGEBREAK!
// Paths
// Copy the next path element from path into name.
// Return a pointer to the element following the copied one.
// The returned path has no leading slashes,
// so the caller can check *path=='\0' to see if the name is the last one.
// If no name to remove, return 0.
//
// Examples:
// skipelem("a/bb/c", name) = "bb/c", setting name = "a"
// skipelem("///a//bb", name) = "bb", setting name = "a"
// skipelem("a", name) = "", setting name = "a"
// skipelem("", name) = skipelem("////", name) = 0
//
static char*
skipelem(char *path, char *name)
{
char *s;
int len;
// Leading slashes
while(*path == '/')
path++;
// End of string
if(*path == 0)
return 0;
s = path;
// Non-slash, non-zero characters
while(*path != '/' && *path != 0)
path++;
len = path - s;
if(len >= DIRSIZ)
memmove(name, s, DIRSIZ);
else {
memmove(name, s, len);
name[len] = 0;
}
// Trailing slashes
while(*path == '/')
path++;
return path;
}
// Look up and return the inode for a path name.
// If parent != 0, return the inode for the parent and copy the final
// path element into name, which must have room for DIRSIZ bytes.
// Must be called inside a transaction since it calls iput().
// Returns 0 on error
static struct inode*
namex(char *path, int nameiparent, char *name)
{
struct inode *ip, *next;
if(*path == '/')
ip = iget(ROOTDEV, ROOTINO);
else
ip = idup(proc->cwd);
// Iterate through path tokens, saving them
// to `name`
while((path = skipelem(path, name)) != 0){
ilock(ip);
if(ip->type != T_DIR){
iunlockput(ip);
return 0;
}
if(nameiparent && *path == '\0'){
// Stop one level early, return
// inode of parent containing target
iunlock(ip);
return ip;
}
if((next = dirlookup(ip, name, 0)) == 0){
// Token not found in current directory
iunlockput(ip);
return 0;
}
iunlockput(ip);
ip = next;
}
if(nameiparent){
// Called with empty path
iput(ip);
return 0;
}
return ip;
}
// Evaluate path and return the corresponding inode.
struct inode*
namei(char *path)
{
char name[DIRSIZ];
return namex(path, 0, name);
}
// Evaluate path and return the inode corresponding
// to the parent directory of the target file, and
// store the final token in the path traversal in
// `name`
struct inode*
nameiparent(char *path, char *name)
{
return namex(path, 1, name);
}