minix/servers/fs/read.c

/* This file contains the heart of the mechanism used to read (and write)
 * files.  Read and write requests are split up into chunks that do not cross
 * block boundaries.  Each chunk is then processed in turn.  Reads on special
 * files are also detected and handled.
 *
 * The entry points into this file are
 *   do_read:	 perform the READ system call by calling read_write
 *   read_write: actually do the work of READ and WRITE
 *   read_map:	 given an inode and file position, look up its zone number
 *   rd_indir:	 read an entry in an indirect block 
 *   read_ahead: manage the block read ahead business
 */

#include "fs.h"
#include <fcntl.h>
#include <minix/com.h>
#include "buf.h"
#include "file.h"
#include "fproc.h"
#include "inode.h"
#include "param.h"
#include "super.h"

#define VVIRCOPY 0

#if VVIRCOPY
#define NOVVIRCOPY	0
#else
#define NOVVIRCOPY	1
#endif

FORWARD _PROTOTYPE( int rw_chunk, (struct inode *rip, off_t position,
			unsigned off, int chunk, unsigned left, int rw_flag,
			char *buff, int seg, int usr, int block_size, int *completed)			);

FORWARD _PROTOTYPE( int rw_chunk_finish, (int *)			);

/*===========================================================================*
 *				do_read					     *
 *===========================================================================*/
PUBLIC int do_read()
{
  return(read_write(READING));
}

/* The following data is shared between rw_chunk() and rw_chunk_finish().
 * It saves up the copying operations that have to be done between user
 * space and the file system. After every (set of) rw_chunk()s,
 * rw_chunk_finish() has to be called immediately (before the FS call returns)
 * so the actual data copying is done.
 *
 * The point of this is to save on the number of copy system calls.
 */
#define COPY_QUEUE_LEN CPVEC_NR 
PRIVATE struct copy_queue_entry {
	struct buf *bp;			/* buf to put_block after copying */
	int user_seg, user_proc;	/* user space data */
	phys_bytes user_offset, fs_offset;
	int blocktype;
	int chunk;
	int op;
} copy_queue[COPY_QUEUE_LEN];
PRIVATE int copy_queue_used = 0;

/*===========================================================================*
 *				read_write				     *
 *===========================================================================*/
PUBLIC int read_write(rw_flag)
int rw_flag;			/* READING or WRITING */
{
/* Perform read(fd, buffer, nbytes) or write(fd, buffer, nbytes) call. */

  register struct inode *rip;
  register struct filp *f;
  off_t bytes_left, f_size, position;
  unsigned int off, cum_io;
  int op, oflags, r, chunk, usr, seg, block_spec, char_spec;
  int regular, partial_pipe = 0, partial_cnt = 0;
  mode_t mode_word;
  struct filp *wf;
  int block_size;
  int completed, r2 = OK;
  phys_bytes p;

  /* left unfinished rw_chunk()s from previous call! this can't happen.
   * it means something has gone wrong we can't repair now.
   */
  if(copy_queue_used != 0) {
  	panic(__FILE__,"start - copy queue size nonzero", copy_queue_used);
  }
  if(bufs_in_use < 0) {
  	panic(__FILE__,"start - bufs_in_use negative", bufs_in_use);
  }

  /* MM loads segments by putting funny things in upper 10 bits of 'fd'. */
  if (who == PM_PROC_NR && (m_in.fd & (~BYTE)) ) {
	usr = m_in.fd >> 7;
	seg = (m_in.fd >> 5) & 03;
	m_in.fd &= 037;		/* get rid of user and segment bits */
  } else {
	usr = who;		/* normal case */
	seg = D;
  }

  /* If the file descriptor is valid, get the inode, size and mode. */
  if (m_in.nbytes < 0) return(EINVAL);
  if ((f = get_filp(m_in.fd)) == NIL_FILP) return(err_code);
  if (((f->filp_mode) & (rw_flag == READING ? R_BIT : W_BIT)) == 0) {
	return(f->filp_mode == FILP_CLOSED ? EIO : EBADF);
  }
  if (m_in.nbytes == 0) return(0);	/* so char special files need not check for 0*/

  /* check if user process has the memory it needs.
   * if not, copying will fail later.
   * do this after 0-check above because umap doesn't want to map 0 bytes.
   */
  if ((r = sys_umap(usr, seg, (vir_bytes) m_in.buffer, m_in.nbytes, &p)) != OK)
	return r;
  position = f->filp_pos;
  oflags = f->filp_flags;
  rip = f->filp_ino;
  f_size = rip->i_size;
  r = OK;
  if (rip->i_pipe == I_PIPE) {
	/* fp->fp_cum_io_partial is only nonzero when doing partial writes */
	cum_io = fp->fp_cum_io_partial; 
  } else {
	cum_io = 0;
  }
  op = (rw_flag == READING ? DEV_READ : DEV_WRITE);
  mode_word = rip->i_mode & I_TYPE;
  regular = mode_word == I_REGULAR || mode_word == I_NAMED_PIPE;

  if((char_spec = (mode_word == I_CHAR_SPECIAL ? 1 : 0))) {
  	if(rip->i_zone[0] == NO_DEV)
  		panic(__FILE__,"read_write tries to read from character device NO_DEV", NO_NUM);
  	block_size = get_block_size(rip->i_zone[0]);
  }
  if((block_spec = (mode_word == I_BLOCK_SPECIAL ? 1 : 0))) {
  	f_size = LONG_MAX;
  	if(rip->i_zone[0] == NO_DEV)
  		panic(__FILE__,"read_write tries to read from block device NO_DEV", NO_NUM);
  	block_size = get_block_size(rip->i_zone[0]);
  }

  if(!char_spec && !block_spec)
  	block_size = rip->i_sp->s_block_size;

  rdwt_err = OK;		/* set to EIO if disk error occurs */

  /* Check for character special files. */
  if (char_spec) {
  	dev_t dev;
	dev = (dev_t) rip->i_zone[0];
	r = dev_io(op, dev, usr, m_in.buffer, position, m_in.nbytes, oflags);
	if (r >= 0) {
		cum_io = r;
		position += r;
		r = OK;
	}
  } else {
	if (rw_flag == WRITING && block_spec == 0) {
		/* Check in advance to see if file will grow too big. */
		if (position > rip->i_sp->s_max_size - m_in.nbytes) 
			return(EFBIG);

		/* Check for O_APPEND flag. */
		if (oflags & O_APPEND) position = f_size;

		/* Clear the zone containing present EOF if hole about
		 * to be created.  This is necessary because all unwritten
		 * blocks prior to the EOF must read as zeros.
		 */
		if (position > f_size) clear_zone(rip, f_size, 0);
	}

	/* Pipes are a little different.  Check. */
	if (rip->i_pipe == I_PIPE) {
		struct filp *other_end;
	       r = pipe_check(rip, rw_flag, oflags,
	       		m_in.nbytes, position, &partial_cnt, 0);
	       if (r <= 0) return(r);
	}

	if (partial_cnt > 0) partial_pipe = 1;

	/* Split the transfer into chunks that don't span two blocks. */
	while (m_in.nbytes != 0) {

		off = (unsigned int) (position % block_size);/* offset in blk*/
		if (partial_pipe) {  /* pipes only */
			chunk = MIN(partial_cnt, block_size - off);
		} else
			chunk = MIN(m_in.nbytes, block_size - off);
		if (chunk < 0) chunk = block_size - off;

		if (rw_flag == READING) {
			bytes_left = f_size - position;
			if (position >= f_size) break;	/* we are beyond EOF */
			if (chunk > bytes_left) chunk = (int) bytes_left;
		}

		/* Read or write 'chunk' bytes. */
		r = rw_chunk(rip, position, off, chunk, (unsigned) m_in.nbytes,
			     rw_flag, m_in.buffer, seg, usr, block_size, &completed);

		if (r != OK) break;	/* EOF reached */
		if (rdwt_err < 0) break;

		/* Update counters and pointers. */
		m_in.buffer += chunk;	/* user buffer address */
		m_in.nbytes -= chunk;	/* bytes yet to be read */
		cum_io += chunk;	/* bytes read so far */
		position += chunk;	/* position within the file */

		if (partial_pipe) {
			partial_cnt -= chunk;
			if (partial_cnt <= 0)  break;
		}
	}
  }

#if VVIRCOPY
  /* do copying to/from user space */
  r2 = rw_chunk_finish(&completed);
#endif

  /* On write, update file size and access time. */
  if (rw_flag == WRITING) {
	if (regular || mode_word == I_DIRECTORY) {
		if (position > f_size) rip->i_size = position;
	}
  } else {
	if (rip->i_pipe == I_PIPE) {
		if( position >= rip->i_size) {
			/* Reset pipe pointers. */
			rip->i_size = 0;	/* no data left */
			position = 0;		/* reset reader(s) */
			wf = find_filp(rip, W_BIT);
			if (wf != NIL_FILP) wf->filp_pos = 0;
		}
	}
  }
  f->filp_pos = position;

  /* Check to see if read-ahead is called for, and if so, set it up. */
  if (rw_flag == READING && rip->i_seek == NO_SEEK && position % block_size== 0
		&& (regular || mode_word == I_DIRECTORY)) {
	rdahed_inode = rip;
	rdahedpos = position;
  }
  rip->i_seek = NO_SEEK;

  if (rdwt_err != OK) r = rdwt_err;	/* check for disk error */
  if (rdwt_err == END_OF_FILE) r = OK;

  /* if user-space copying failed, read/write failed. */
  if (r == OK && r2 != OK) {
	r = r2;
  }
  if (r == OK) {
	if (rw_flag == READING) rip->i_update |= ATIME;
	if (rw_flag == WRITING) rip->i_update |= CTIME | MTIME;
	rip->i_dirt = DIRTY;		/* inode is thus now dirty */
	if (partial_pipe) {
		partial_pipe = 0;
			/* partial write on pipe with */
		/* O_NONBLOCK, return write count */
		if (!(oflags & O_NONBLOCK)) {
			fp->fp_cum_io_partial = cum_io;
			suspend(XPIPE);   /* partial write on pipe with */
			return(SUSPEND);  /* nbyte > PIPE_SIZE - non-atomic */
		}
	}
	fp->fp_cum_io_partial = 0;
	return(cum_io);
  }
  if(copy_queue_used != 0) {
  	panic(__FILE__,"end - copy queue size nonzero", copy_queue_used);
  }
  if(bufs_in_use < 0) {
  	panic(__FILE__,"end - bufs_in_use negative", bufs_in_use);
  }
  return(r);
}

/*===========================================================================*
 *				rw_chunk				     *
 *===========================================================================*/
PRIVATE int rw_chunk(rip, position, off, chunk, left, rw_flag, buff, seg, usr, block_size, completed)
register struct inode *rip;	/* pointer to inode for file to be rd/wr */
off_t position;			/* position within file to read or write */
unsigned off;			/* off within the current block */
int chunk;			/* number of bytes to read or write */
unsigned left;			/* max number of bytes wanted after position */
int rw_flag;			/* READING or WRITING */
char *buff;			/* virtual address of the user buffer */
int seg;			/* T or D segment in user space */
int usr;			/* which user process */
int block_size;			/* block size of FS operating on */
int *completed;			/* number of bytes copied */
{
/* Read or write (part of) a block. */

  register struct buf *bp;
  register int r = OK;
  int n, block_spec;
  block_t b;
  dev_t dev;
  int entry;

  *completed = 0;

  block_spec = (rip->i_mode & I_TYPE) == I_BLOCK_SPECIAL;
  if (block_spec) {
	b = position/block_size;
	dev = (dev_t) rip->i_zone[0];
  } else {
	b = read_map(rip, position);
	dev = rip->i_dev;
  }

  if (!block_spec && b == NO_BLOCK) {
	if (rw_flag == READING) {
		/* Reading from a nonexistent block.  Must read as all zeros.*/
		bp = get_block(NO_DEV, NO_BLOCK, NORMAL);    /* get a buffer */
		zero_block(bp);
	} else {
		/* Writing to a nonexistent block. Create and enter in inode.*/
		if ((bp= new_block(rip, position)) == NIL_BUF)return(err_code);
	}
  } else if (rw_flag == READING) {
	/* Read and read ahead if convenient. */
	bp = rahead(rip, b, position, left);
  } else {
	/* Normally an existing block to be partially overwritten is first read
	 * in.  However, a full block need not be read in.  If it is already in
	 * the cache, acquire it, otherwise just acquire a free buffer.
	 */
	n = (chunk == block_size ? NO_READ : NORMAL);
	if (!block_spec && off == 0 && position >= rip->i_size) n = NO_READ;
	bp = get_block(dev, b, n);
  }

  /* In all cases, bp now points to a valid buffer. */
  if(bp == NIL_BUF) {
  	panic(__FILE__,"bp not valid in rw_chunk, this can't happen", NO_NUM);
  }
  if (rw_flag == WRITING && chunk != block_size && !block_spec &&
					position >= rip->i_size && off == 0) {
	zero_block(bp);
  }

#if NOVVIRCOPY
  if (rw_flag == READING) {
	/* Copy a chunk from the block buffer to user space. */
	r = sys_vircopy(FS_PROC_NR, D, (phys_bytes) (bp->b_data+off),
			usr, seg, (phys_bytes) buff,
			(phys_bytes) chunk);
  } else {
	/* Copy a chunk from user space to the block buffer. */
	r = sys_vircopy(usr, seg, (phys_bytes) buff,
			FS_PROC_NR, D, (phys_bytes) (bp->b_data+off),
			(phys_bytes) chunk);
	bp->b_dirt = DIRTY;
  }
  n = (off + chunk == block_size ? FULL_DATA_BLOCK : PARTIAL_DATA_BLOCK);
  put_block(bp, n);
#else
  /* have to copy a buffer now. remember to do it. */
  if(copy_queue_used < 0 || copy_queue_used > COPY_QUEUE_LEN) {
  	panic(__FILE__,"copy_queue_used illegal size", copy_queue_used);
  }

  if(copy_queue_used == COPY_QUEUE_LEN) {
  	r = rw_chunk_finish(completed);
	if(copy_queue_used != 0) {
	  	panic(__FILE__,"copy_queue_used nonzero", copy_queue_used);
  	}
  }

  entry = copy_queue_used++;

  if(entry < 0 || entry >= COPY_QUEUE_LEN) {
  	panic(__FILE__,"entry illegal slot", entry);
  }

  copy_queue[entry].bp = bp;
  copy_queue[entry].op = rw_flag;
  copy_queue[entry].user_seg = seg;
  copy_queue[entry].user_proc = usr;
  copy_queue[entry].fs_offset = (phys_bytes) bp->b_data+off;
  copy_queue[entry].user_offset = (phys_bytes) buff;
  copy_queue[entry].chunk = chunk;
  copy_queue[entry].blocktype = (off + chunk == block_size ? FULL_DATA_BLOCK : PARTIAL_DATA_BLOCK);

  if(rw_flag == WRITING)  {
	bp->b_dirt = DIRTY;
  }
#endif

  return(r);
}

/*===========================================================================*
 *				rw_chunk_finish				     *
 *===========================================================================*/
PRIVATE int rw_chunk_finish(int *completed)
{
	int i, total = 0, r;
	static struct vir_cp_req vir_cp_req[CPVEC_NR];
	message m;

	*completed = 0;
	if(copy_queue_used < 1) {
		return OK;
	}

	for(i = 0; i < copy_queue_used; i++) {
		struct vir_addr *fs, *user;

		if(copy_queue[i].op == READING) {
			fs = &vir_cp_req[i].src;
			user = &vir_cp_req[i].dst;
		} else {
			fs = &vir_cp_req[i].dst;
			user = &vir_cp_req[i].src;
		}

		vir_cp_req[i].count = copy_queue[i].chunk;
		fs->proc_nr = FS_PROC_NR;
		fs->segment = D;
		fs->offset = copy_queue[i].fs_offset;
		user->proc_nr = copy_queue[i].user_proc;
		user->segment = copy_queue[i].user_seg;
		user->offset = copy_queue[i].user_offset;
		total += copy_queue[i].chunk;
	}

	m.m_type = SYS_VIRVCOPY;
	m.VCP_VEC_SIZE = i;
	m.VCP_VEC_ADDR = (char *) vir_cp_req;

	if((r=sendrec(SYSTASK, &m)) < 0) {
		panic(__FILE__,"rw_chunk_finish: virvcopy sendrec failed", r);
	}

	for(i = 0; i < copy_queue_used; i++) {
		put_block(copy_queue[i].bp, copy_queue[i].blocktype);
	}

	*completed = total;

  	copy_queue_used = 0;

	/* return VIRVCOPY return code */
	return m.m_type;
}

/*===========================================================================*
 *				read_map				     *
 *===========================================================================*/
PUBLIC block_t read_map(rip, position)
register struct inode *rip;	/* ptr to inode to map from */
off_t position;			/* position in file whose blk wanted */
{
/* Given an inode and a position within the corresponding file, locate the
 * block (not zone) number in which that position is to be found and return it.
 */

  register struct buf *bp;
  register zone_t z;
  int scale, boff, dzones, nr_indirects, index, zind, ex;
  block_t b;
  long excess, zone, block_pos;
  
  scale = rip->i_sp->s_log_zone_size;	/* for block-zone conversion */
  block_pos = position/rip->i_sp->s_block_size;	/* relative blk # in file */
  zone = block_pos >> scale;	/* position's zone */
  boff = (int) (block_pos - (zone << scale) ); /* relative blk # within zone */
  dzones = rip->i_ndzones;
  nr_indirects = rip->i_nindirs;

  /* Is 'position' to be found in the inode itself? */
  if (zone < dzones) {
	zind = (int) zone;	/* index should be an int */
	z = rip->i_zone[zind];
	if (z == NO_ZONE) return(NO_BLOCK);
	b = ((block_t) z << scale) + boff;
	return(b);
  }

  /* It is not in the inode, so it must be single or double indirect. */
  excess = zone - dzones;	/* first Vx_NR_DZONES don't count */

  if (excess < nr_indirects) {
	/* 'position' can be located via the single indirect block. */
	z = rip->i_zone[dzones];
  } else {
	/* 'position' can be located via the double indirect block. */
	if ( (z = rip->i_zone[dzones+1]) == NO_ZONE) return(NO_BLOCK);
	excess -= nr_indirects;			/* single indir doesn't count*/
	b = (block_t) z << scale;
	bp = get_block(rip->i_dev, b, NORMAL);	/* get double indirect block */
	index = (int) (excess/nr_indirects);
	z = rd_indir(bp, index);		/* z= zone for single*/
	put_block(bp, INDIRECT_BLOCK);		/* release double ind block */
	excess = excess % nr_indirects;		/* index into single ind blk */
  }

  /* 'z' is zone num for single indirect block; 'excess' is index into it. */
  if (z == NO_ZONE) return(NO_BLOCK);
  b = (block_t) z << scale;			/* b is blk # for single ind */
  bp = get_block(rip->i_dev, b, NORMAL);	/* get single indirect block */
  ex = (int) excess;				/* need an integer */
  z = rd_indir(bp, ex);				/* get block pointed to */
  put_block(bp, INDIRECT_BLOCK);		/* release single indir blk */
  if (z == NO_ZONE) return(NO_BLOCK);
  b = ((block_t) z << scale) + boff;
  return(b);
}


/*===========================================================================*
 *				rd_indir				     *
 *===========================================================================*/
PUBLIC zone_t rd_indir(bp, index)
struct buf *bp;			/* pointer to indirect block */
int index;			/* index into *bp */
{
/* Given a pointer to an indirect block, read one entry.  The reason for
 * making a separate routine out of this is that there are four cases:
 * V1 (IBM and 68000), and V2 (IBM and 68000).
 */

  struct super_block *sp;
  zone_t zone;			/* V2 zones are longs (shorts in V1) */

  sp = get_super(bp->b_dev);	/* need super block to find file sys type */

  /* read a zone from an indirect block */
  if (sp->s_version == V1)
	zone = (zone_t) conv2(sp->s_native, (int)  bp->b_v1_ind[index]);
  else
	zone = (zone_t) conv4(sp->s_native, (long) bp->b_v2_ind[index]);

  if (zone != NO_ZONE &&
		(zone < (zone_t) sp->s_firstdatazone || zone >= sp->s_zones)) {
	printf("Illegal zone number %ld in indirect block, index %d\n",
	       (long) zone, index);
	panic(__FILE__,"check file system", NO_NUM);
  }
  return(zone);
}


/*===========================================================================*
 *				read_ahead				     *
 *===========================================================================*/
PUBLIC void read_ahead()
{
/* Read a block into the cache before it is needed. */
  int block_size;
  register struct inode *rip;
  struct buf *bp;
  block_t b;

  rip = rdahed_inode;		/* pointer to inode to read ahead from */
  block_size = get_block_size(rip->i_dev);
  rdahed_inode = NIL_INODE;	/* turn off read ahead */
  if ( (b = read_map(rip, rdahedpos)) == NO_BLOCK) return;	/* at EOF */
  bp = rahead(rip, b, rdahedpos, block_size);
  put_block(bp, PARTIAL_DATA_BLOCK);
}


/*===========================================================================*
 *				rahead					     *
 *===========================================================================*/
PUBLIC struct buf *rahead(rip, baseblock, position, bytes_ahead)
register struct inode *rip;	/* pointer to inode for file to be read */
block_t baseblock;		/* block at current position */
off_t position;			/* position within file */
unsigned bytes_ahead;		/* bytes beyond position for immediate use */
{
/* Fetch a block from the cache or the device.  If a physical read is
 * required, prefetch as many more blocks as convenient into the cache.
 * This usually covers bytes_ahead and is at least BLOCKS_MINIMUM.
 * The device driver may decide it knows better and stop reading at a
 * cylinder boundary (or after an error).  Rw_scattered() puts an optional
 * flag on all reads to allow this.
 */
  int block_size;
/* Minimum number of blocks to prefetch. */
# define BLOCKS_MINIMUM		(NR_BUFS < 50 ? 18 : 32)
  int block_spec, scale, read_q_size;
  unsigned int blocks_ahead, fragment;
  block_t block, blocks_left;
  off_t ind1_pos;
  dev_t dev;
  struct buf *bp;
  static struct buf *read_q[NR_BUFS];

  block_spec = (rip->i_mode & I_TYPE) == I_BLOCK_SPECIAL;
  if (block_spec) {
	dev = (dev_t) rip->i_zone[0];
  } else {
	dev = rip->i_dev;
  }
  block_size = get_block_size(dev);

  block = baseblock;
  bp = get_block(dev, block, PREFETCH);
  if (bp->b_dev != NO_DEV) return(bp);

  /* The best guess for the number of blocks to prefetch:  A lot.
   * It is impossible to tell what the device looks like, so we don't even
   * try to guess the geometry, but leave it to the driver.
   *
   * The floppy driver can read a full track with no rotational delay, and it
   * avoids reading partial tracks if it can, so handing it enough buffers to
   * read two tracks is perfect.  (Two, because some diskette types have
   * an odd number of sectors per track, so a block may span tracks.)
   *
   * The disk drivers don't try to be smart.  With todays disks it is
   * impossible to tell what the real geometry looks like, so it is best to
   * read as much as you can.  With luck the caching on the drive allows
   * for a little time to start the next read.
   *
   * The current solution below is a bit of a hack, it just reads blocks from
   * the current file position hoping that more of the file can be found.  A
   * better solution must look at the already available zone pointers and
   * indirect blocks (but don't call read_map!).
   */

  fragment = position % block_size;
  position -= fragment;
  bytes_ahead += fragment;

  blocks_ahead = (bytes_ahead + block_size - 1) / block_size;

  if (block_spec && rip->i_size == 0) {
	blocks_left = NR_IOREQS;
  } else {
	blocks_left = (rip->i_size - position + block_size - 1) / block_size;

	/* Go for the first indirect block if we are in its neighborhood. */
	if (!block_spec) {
		scale = rip->i_sp->s_log_zone_size;
		ind1_pos = (off_t) rip->i_ndzones * (block_size << scale);
		if (position <= ind1_pos && rip->i_size > ind1_pos) {
			blocks_ahead++;
			blocks_left++;
		}
	}
  }

  /* No more than the maximum request. */
  if (blocks_ahead > NR_IOREQS) blocks_ahead = NR_IOREQS;

  /* Read at least the minimum number of blocks, but not after a seek. */
  if (blocks_ahead < BLOCKS_MINIMUM && rip->i_seek == NO_SEEK)
	blocks_ahead = BLOCKS_MINIMUM;

  /* Can't go past end of file. */
  if (blocks_ahead > blocks_left) blocks_ahead = blocks_left;

  read_q_size = 0;

  /* Acquire block buffers. */
  for (;;) {
	read_q[read_q_size++] = bp;

	if (--blocks_ahead == 0) break;

	/* Don't trash the cache, leave 4 free. */
	if (bufs_in_use >= NR_BUFS - 4) break;

	block++;

	bp = get_block(dev, block, PREFETCH);
	if (bp->b_dev != NO_DEV) {
		/* Oops, block already in the cache, get out. */
		put_block(bp, FULL_DATA_BLOCK);
		break;
	}
  }
  rw_scattered(dev, read_q, read_q_size, READING);
  return(get_block(dev, baseblock, NORMAL));
}