Minix2.0/src/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 FD_MASK 077 /* max file descriptor is 63 */
PRIVATE message umess; /* message for asking SYSTASK for user copy */
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) );
/*===========================================================================*
* 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;
dev_t dev;
mode_t mode_word;
struct filp *wf;
/* MM loads segments by putting funny things in upper 10 bits of 'fd'. */
if (who == MM_PROC_NR && (fd & (~BYTE)) ) {
usr = (fd >> 8) & BYTE;
seg = (fd >> 6) & 03;
fd &= FD_MASK; /* 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 (nbytes < 0) return(EINVAL);
if ((f = get_filp(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 (nbytes == 0) return(0); /* so char special files need not check for 0*/
position = f->filp_pos;
if (position > MAX_FILE_POS) return(EINVAL);
if (position + nbytes < position) return(EINVAL); /* unsigned overflow */
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;
char_spec = (mode_word == I_CHAR_SPECIAL ? 1 : 0);
block_spec = (mode_word == I_BLOCK_SPECIAL ? 1 : 0);
if (block_spec) f_size = LONG_MAX;
rdwt_err = OK; /* set to EIO if disk error occurs */
/* Check for character special files. */
if (char_spec) {
dev = (dev_t) rip->i_zone[0];
r = dev_io(op, oflags & O_NONBLOCK, dev, position, nbytes, who,buffer);
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 - 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,nbytes,position,&partial_cnt);
if (r <= 0) return(r);
}
if (partial_cnt > 0) partial_pipe = 1;
/* Split the transfer into chunks that don't span two blocks. */
while (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(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) nbytes,
rw_flag, buffer, seg, usr);
if (r != OK) break; /* EOF reached */
if (rdwt_err < 0) break;
/* Update counters and pointers. */
buffer += chunk; /* user buffer address */
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 && position >= rip->i_size) {
/* Reset pipe pointers. */
rip->i_size = 0; /* no data left */
position = 0; /* reset reader(s) */
if ( (wf = find_filp(rip, W_BIT)) != 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 (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(0); /* nbyte > PIPE_SIZE - non-atomic */
}
}
fp->fp_cum_io_partial = 0;
return(cum_io);
} else {
return(r);
}
}
/*===========================================================================*
* rw_chunk *
*===========================================================================*/
PRIVATE int rw_chunk(rip, position, off, chunk, left, rw_flag, buff, seg, usr)
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 */
{
/* Read or write (part of) a block. */
register struct buf *bp;
register int r;
int n, block_spec;
block_t b;
dev_t dev;
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 (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_copy(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_copy(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/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("check file system", NO_NUM);
}
return(zone);
}
/*===========================================================================*
* read_ahead *
*===========================================================================*/
PUBLIC void read_ahead()
{
/* Read a block into the cache before it is needed. */
register struct inode *rip;
struct buf *bp;
block_t b;
rip = rdahed_inode; /* pointer to inode to read ahead from */
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.
*/
/* 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 = 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));
}