clarify some FS comments

Author: Robert Morris

bio.c

@@ -7,7 +7,7 @@
 // 
 // Interface:
 // * To get a buffer for a particular disk block, call bread.
-// * After changing buffer data, call bwrite to flush it to disk.
+// * After changing buffer data, call bwrite to write it to disk.
 // * When done with the buffer, call brelse.
 // * Do not use the buffer after calling brelse.
 // * Only one process at a time can use a buffer,
@@ -16,8 +16,7 @@
 // The implementation uses three state flags internally:
 // * B_BUSY: the block has been returned from bread
 //     and has not been passed back to brelse.  
-// * B_VALID: the buffer data has been initialized
-//     with the associated disk block contents.
+// * B_VALID: the buffer data has been read from the disk.
 // * B_DIRTY: the buffer data has been modified
 //     and needs to be written to disk.
 
@@ -58,7 +57,7 @@ binit(void)
 
 // Look through buffer cache for sector on device dev.
 // If not found, allocate fresh block.
-// In either case, return locked buffer.
+// In either case, return B_BUSY buffer.
 static struct buf*
 bget(uint dev, uint sector)
 {
@@ -67,7 +66,7 @@ bget(uint dev, uint sector)
   acquire(&bcache.lock);
 
  loop:
-  // Try for cached block.
+  // Is the sector already cached?
   for(b = bcache.head.next; b != &bcache.head; b = b->next){
     if(b->dev == dev && b->sector == sector){
       if(!(b->flags & B_BUSY)){
@@ -80,7 +79,7 @@ bget(uint dev, uint sector)
     }
   }
 
-  // Allocate fresh block.
+  // Not cached; recycle some existing buffer.
   for(b = bcache.head.prev; b != &bcache.head; b = b->prev){
     if((b->flags & B_BUSY) == 0){
       b->dev = dev;
@@ -105,7 +104,7 @@ bread(uint dev, uint sector)
   return b;
 }
 
-// Write b's contents to disk.  Must be locked.
+// Write b's contents to disk.  Must be B_BUSY.
 void
 bwrite(struct buf *b)
 {
@@ -115,7 +114,8 @@ bwrite(struct buf *b)
   iderw(b);
 }
 
-// Release the buffer b.
+// Release a B_BUSY buffer.
+// Move to the head of the MRU list.
 void
 brelse(struct buf *b)
 {

file.h

@@ -9,8 +9,7 @@ struct file {
 };
 
 
-// in-core file system types
-
+// in-memory copy of an inode
 struct inode {
   uint dev;           // Device number
   uint inum;          // Inode number
@@ -24,12 +23,11 @@ struct inode {
   uint size;
   uint addrs[NDIRECT+1];
 };
-
 #define I_BUSY 0x1
 #define I_VALID 0x2
 
-// device implementations
-
+// table mapping major device number to
+// device functions
 struct devsw {
   int (*read)(struct inode*, char*, int);
   int (*write)(struct inode*, char*, int);

fs.c

@@ -1,11 +1,10 @@
-// File system implementation.  Four layers:
+// 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.
 //
-// Disk layout is: superblock, inodes, block in-use bitmap, data blocks.
-//
 // This file contains the low-level file system manipulation 
 // routines.  The (higher-level) system call implementations
 // are in sysfile.c.
@@ -61,10 +60,10 @@ balloc(uint dev)
   readsb(dev, &sb);
   for(b = 0; b < sb.size; b += BPB){
     bp = bread(dev, BBLOCK(b, sb.ninodes));
-    for(bi = 0; bi < BPB && bi < (sb.size - b); bi++){
+    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 on disk.
+        bp->data[bi/8] |= m;  // Mark block in use.
         log_write(bp);
         brelse(bp);
         bzero(dev, b + bi);
@@ -90,22 +89,27 @@ bfree(int dev, uint b)
   m = 1 << (bi % 8);
   if((bp->data[bi/8] & m) == 0)
     panic("freeing free block");
-  bp->data[bi/8] &= ~m;  // Mark block free on disk.
+  bp->data[bi/8] &= ~m;
   log_write(bp);
   brelse(bp);
 }
 
 // Inodes.
 //
-// An inode is a single, unnamed file in the file system.
-// The inode disk structure holds metadata (the type, device numbers,
-// and data size) along with a list of blocks where the associated
-// data can be found.
+// 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 immediately after
-// the superblock.  The kernel keeps a cache of the in-use
-// on-disk structures to provide a place for synchronizing access
-// to inodes shared between multiple processes.
+// the superblock. 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.
 // 
 // ip->ref counts the number of pointer references to this cached
 // inode; references are typically kept in struct file and in proc->cwd.
@@ -114,11 +118,12 @@ bfree(int dev, uint b)
 //
 // Processes are only allowed to read and write inode
 // metadata and contents when holding the inode's lock,
-// represented by the I_BUSY flag in the in-memory copy.
+// represented by the I_BUSY bit in ip->flags.
 // Because inode locks are held during disk accesses, 
 // they are implemented using a flag rather than with
-// spin locks.  Callers are responsible for locking
-// inodes before passing them to routines in this file; leaving
+// spin locks.  ilock() and iunlock() manipulate an
+// inode's I_BUSY flag. Many routines in this file expect
+// the caller to have already locked the inode; leaving
 // this responsibility with the caller makes it possible for them
 // to create arbitrarily-sized atomic operations.
 //
@@ -127,6 +132,19 @@ bfree(int dev, uint b)
 // return pointers to *unlocked* inodes.  It is the callers'
 // responsibility to lock them before using them.  A non-zero
 // ip->ref keeps these unlocked inodes in the cache.
+//
+// In order for the file system code to look at an inode, the inode
+// must pass through a number of states, with transitions
+// driven by the indicated functions:
+//  
+// * Allocated on disk, indicated by a non-zero type.
+//   ialloc() and iput().
+// * Referenced in the cache, indicated by ip->ref > 0.
+//   iget() and iput().
+// * Cached inode is valid, indicated by I_VALID.
+//   ilock() and iput().
+// * Locked, indicated by I_BUSY.
+//   ilock() and iunlock().
 
 struct {
   struct spinlock lock;
@@ -143,6 +161,7 @@ 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)
 {
@@ -152,7 +171,8 @@ ialloc(uint dev, short type)
   struct superblock sb;
 
   readsb(dev, &sb);
-  for(inum = 1; inum < sb.ninodes; inum++){  // loop over inode blocks
+
+  for(inum = 1; inum < sb.ninodes; inum++){
     bp = bread(dev, IBLOCK(inum));
     dip = (struct dinode*)bp->data + inum%IPB;
     if(dip->type == 0){  // a free inode

fs.h

@@ -1,8 +1,12 @@
 // On-disk file system format. 
 // Both the kernel and user programs use this header file.
 
-// Block 0 is unused.  Block 1 is super block.
-// Inodes start at block 2.
+// Block 0 is unused.
+// Block 1 is super block.
+// Blocks 2 through sb.ninodes/IPB hold inodes.
+// Then free bitmap blocks holding sb.size bits.
+// Then sb.nblocks data blocks.
+// Then sb.nlog log blocks.
 
 #define ROOTINO 1  // root i-number
 #define BSIZE 512  // block size

ide.c

@@ -93,7 +93,7 @@ ideintr(void)
 {
   struct buf *b;
 
-  // Take first buffer off queue.
+  // First queued buffer is the active request.
   acquire(&idelock);
   if((b = idequeue) == 0){
     release(&idelock);
@@ -134,11 +134,11 @@ iderw(struct buf *b)
   if(b->dev != 0 && !havedisk1)
     panic("iderw: ide disk 1 not present");
 
-  acquire(&idelock);  // DOC:acquire-lock
+  acquire(&idelock);  //DOC: acquire-lock
 
   // Append b to idequeue.
   b->qnext = 0;
-  for(pp=&idequeue; *pp; pp=&(*pp)->qnext)  // DOC:insert-queue
+  for(pp=&idequeue; *pp; pp=&(*pp)->qnext)  //DOC: insert-queue
     ;
   *pp = b;
 
@@ -147,7 +147,6 @@ iderw(struct buf *b)
     idestart(b);
 
   // Wait for request to finish.
-  // Assuming will not sleep too long: ignore proc->killed.
   while((b->flags & (B_VALID|B_DIRTY)) != B_VALID){
     sleep(b, &idelock);
   }

log.c

@@ -42,7 +42,7 @@ struct log {
   struct spinlock lock;
   int start;
   int size;
-  int intrans;
+  int busy; // a transaction is active
   int dev;
   struct logheader lh;
 };
@@ -75,7 +75,7 @@ install_trans(void)
     struct buf *lbuf = bread(log.dev, log.start+tail+1); // read log block
     struct buf *dbuf = bread(log.dev, log.lh.sector[tail]); // read dst
     memmove(dbuf->data, lbuf->data, BSIZE);  // copy block to dst
-    bwrite(dbuf);  // flush dst to disk
+    bwrite(dbuf);  // write dst to disk
     brelse(lbuf); 
     brelse(dbuf);
   }
@@ -95,7 +95,9 @@ read_head(void)
   brelse(buf);
 }
 
-// Write in-memory log header to disk, committing log entries till head
+// Write in-memory log header to disk.
+// This is the true point at which the
+// current transaction commits.
 static void
 write_head(void)
 {
@@ -123,25 +125,25 @@ void
 begin_trans(void)
 {
   acquire(&log.lock);
-  while (log.intrans) {
+  while (log.busy) {
     sleep(&log, &log.lock);
   }
-  log.intrans = 1;
+  log.busy = 1;
   release(&log.lock);
 }
 
 void
 commit_trans(void)
 {
   if (log.lh.n > 0) {
-    write_head();    // Causes all blocks till log.head to be commited
-    install_trans(); // Install all the transactions till head
+    write_head();    // Write header to disk -- the real commit
+    install_trans(); // Now install writes to home locations
     log.lh.n = 0; 
-    write_head();    // Reclaim log
+    write_head();    // Erase the transaction from the log
   }
 
   acquire(&log.lock);
-  log.intrans = 0;
+  log.busy = 0;
   wakeup(&log);
   release(&log.lock);
 }
@@ -161,7 +163,7 @@ log_write(struct buf *b)
 
   if (log.lh.n >= LOGSIZE || log.lh.n >= log.size - 1)
     panic("too big a transaction");
-  if (!log.intrans)
+  if (!log.busy)
     panic("write outside of trans");
 
   for (i = 0; i < log.lh.n; i++) {