source: trunk/minix/servers/fs/read.c@ 21

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

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));

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

/*===========================================================================*
 *                              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;

  /* PM loads segments by putting funny things in other bits of the
   * message, indicated by a high bit in fd.
   */
  if (who_e == PM_PROC_NR && (m_in.fd & _PM_SEG_FLAG)) {
        seg = (int) m_in.m1_p2;
        usr = (int) m_in.m1_p3;
        m_in.fd &= ~(_PM_SEG_FLAG);     /* get rid of flag bit */
  } else {
        usr = who_e;            /* 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) {
        printf("FS: read_write: umap failed for process %d\n", usr);
        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 = ULONG_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) {
               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;
                }
        }
  }

  /* 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);
  }
  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;

  *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 (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);

  return(r);
}


/*===========================================================================*
 *                              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) */

  if(bp == NIL_BUF)
        panic(__FILE__, "rd_indir() on NIL_BUF", NO_NUM);

  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));
}