Minixファイルシステム読書ノートのget_block

PUBLIC struct buf *get_block(dev, block, only_search)
register dev_t dev;		/* on which device is the block? */
register block_t block;		/* which block is wanted? */
int only_search;		/* if NO_READ, don't read, else act normal */
{
/* Check to see if the requested block is in the block cache.  If so, return
 * a pointer to it.  If not, evict some other block and fetch it (unless
 * 'only_search' is 1).  All the blocks in the cache that are not in use
 * are linked together in a chain, with 'front' pointing to the least recently
 * used block and 'rear' to the most recently used block.  If 'only_search' is
 * 1, the block being requested will be overwritten in its entirety, so it is
 * only necessary to see if it is in the cache; if it is not, any free buffer
 * will do.  It is not necessary to actually read the block in from disk.
 * If 'only_search' is PREFETCH, the block need not be read from the disk,
 * and the device is not to be marked on the block, so callers can tell if
 * the block returned is valid.
 * In addition to the LRU chain, there is also a hash chain to link together
 * blocks whose block numbers end with the same bit strings, for fast lookup.
 */

  int b;
  register struct buf *bp, *prev_ptr;

  /* Search the hash chain for (dev, block). Do_read() can use 
   * get_block(NO_DEV ...) to get an unnamed block to fill with zeros when
   * someone wants to read from a hole in a file, in which case this search
   * is skipped
   */
  if (dev != NO_DEV) {
	b = (int) block & HASH_MASK; // 
	bp = buf_hash[b];            // 
	while (bp != NIL_BUF) {       // 
		if (bp->b_blocknr == block && bp->b_dev == dev) {
			/* Block needed has been found. */
			if (bp->b_count == 0) rm_lru(bp);
			bp->b_count++;	/* record that block is in use */
			return(bp);  // 
		} else {
			/* This block is not the one sought. */
			bp = bp->b_hash; /* move to next block on hash chain */
		}
	}
  }

  /* Desired block is not on available chain.  Take oldest block ('front'). */
  if ((bp = front) == NIL_BUF) panic("all buffers in use", NR_BUFS);
  rm_lru(bp);

  /* Remove the block that was just taken from its hash chain. */// bp 

  b = (int) bp->b_blocknr & HASH_MASK;
  prev_ptr = buf_hash[b];
  if (prev_ptr == bp) {
	buf_hash[b] = bp->b_hash;
  } else {
	/* The block just taken is not on the front of its hash chain. */
	while (prev_ptr->b_hash != NIL_BUF)
		if (prev_ptr->b_hash == bp) {
			prev_ptr->b_hash = bp->b_hash;	/* found it */
			break;
		} else {
			prev_ptr = prev_ptr->b_hash;	/* keep looking */
		}
  }

  /* If the block taken is dirty, make it clean by writing it to the disk.
   * Avoid hysteresis by flushing all other dirty blocks for the same device.
   */
  if (bp->b_dev != NO_DEV) {
	if (bp->b_dirt == DIRTY) flushall(bp->b_dev); // bp ， 
#if ENABLE_CACHE2
	put_block2(bp);
#endif
  }

  /* Fill in block's parameters and add it to the hash chain where it goes. */
  bp->b_dev = dev;		/* fill in device number */
  bp->b_blocknr = block;	/* fill in block number */
  bp->b_count++;		/* record that block is being used */
  b = (int) bp->b_blocknr & HASH_MASK;
  bp->b_hash = buf_hash[b];
  buf_hash[b] = bp;		/* add to hash list */

  /* Go get the requested block unless searching or prefetching. */
  if (dev != NO_DEV) {
#if ENABLE_CACHE2
	if (get_block2(bp, only_search)) /* in 2nd level cache */;
	else
#endif
	if (only_search == PREFETCH) bp->b_dev = NO_DEV;
	else
	if (only_search == NORMAL) rw_block(bp, READING);
  }
  return(bp);			/* return the newly acquired block */
}

PUBLIC void put_block(bp, block_type)
register struct buf *bp;	/* pointer to the buffer to be released */
int block_type;			/* INODE_BLOCK, DIRECTORY_BLOCK, or whatever */
{
/* Return a block to the list of available blocks.   Depending on 'block_type'
 * it may be put on the front or rear of the LRU chain.  Blocks that are
 * expected to be needed again shortly (e.g., partially full data blocks)
 * go on the rear; blocks that are unlikely to be needed again shortly
 * (e.g., full data blocks) go on the front.  Blocks whose loss can hurt
 * the integrity of the file system (e.g., inode blocks) are written to
 * disk immediately if they are dirty.
 */

  if (bp == NIL_BUF) return;	/* it is easier to check here than in caller */

  bp->b_count--;		/* there is one use fewer now */
  if (bp->b_count != 0) return;	/* block is still in use */

  bufs_in_use--;		/* one fewer block buffers in use */

  /* Put this block back on the LRU chain.  If the ONE_SHOT bit is set in
   * 'block_type', the block is not likely to be needed again shortly, so put
   * it on the front of the LRU chain where it will be the first one to be
   * taken when a free buffer is needed later.
   */
  if (block_type & ONE_SHOT) {
	/* Block probably won't be needed quickly. Put it on front of chain.
  	 * It will be the next block to be evicted from the cache.
  	 */
	bp->b_prev = NIL_BUF;
	bp->b_next = front;
	if (front == NIL_BUF)
		rear = bp;	/* LRU chain was empty */
	else
		front->b_prev = bp;
	front = bp;
  } else {
	/* Block probably will be needed quickly.  Put it on rear of chain.
  	 * It will not be evicted from the cache for a long time.
  	 */
	bp->b_prev = rear;
	bp->b_next = NIL_BUF;
	if (rear == NIL_BUF)
		front = bp;
	else
		rear->b_next = bp;
	rear = bp;
  }

  /* Some blocks are so important (e.g., inodes, indirect blocks) that they
   * should be written to the disk immediately to avoid messing up the file
   * system in the event of a crash.
   */
  if ((block_type & WRITE_IMMED) && bp->b_dirt==DIRTY && bp->b_dev != NO_DEV)
	rw_block(bp, WRITING);
}