115 lines
3.2 KiB
C
115 lines
3.2 KiB
C
/* The file system maintains a buffer cache to reduce the number of disk
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* accesses needed. Whenever a read or write to the disk is done, a check is
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* first made to see if the block is in the cache. This file manages the
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* cache.
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*
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* The entry points into this file are:
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* get_block: request to fetch a block for reading or writing from cache
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* put_block: return a block previously requested with get_block
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*
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* Private functions:
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* read_block: read physically the block
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*/
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#include "inc.h"
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#include <minix/com.h>
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#include <minix/u64.h>
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#include <minix/bdev.h>
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#include "buf.h"
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static int read_block(struct buf *);
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struct buf *bp_to_pickup = buf; /* This is a pointer to the next node in the
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* buffer cache to pick up*/
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/*===========================================================================*
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* get_block *
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*===========================================================================*/
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struct buf *get_block(block)
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register block_t block; /* which block is wanted? */
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{
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register struct buf *bp, *free_bp;
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free_bp = NULL;
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/* Find if the block is already loaded */
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for (bp = &buf[0]; bp < &buf[NR_BUFS]; bp++)
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if (bp->b_blocknr == block) {
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/* Block found. Increment count and return it */
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bp->b_count++;
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return bp;
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} else
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if (bp == bp_to_pickup) {
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if (bp->b_count == 0)
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free_bp = bp;
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else /* Increment the node to pickup */
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if (bp_to_pickup < &buf[NR_BUFS] - 1)
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bp_to_pickup++;
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else
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bp_to_pickup = buf;
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}
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if (free_bp == NULL &&
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bp_to_pickup == buf &&
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bp_to_pickup->b_count == 0)
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free_bp = bp_to_pickup;
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if (free_bp != NULL) {
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/* Set fields of data structure */
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free_bp->b_blocknr = block;
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if (read_block(free_bp) != OK) return NULL;
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free_bp->b_count = 1;
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if (bp_to_pickup < &buf[NR_BUFS] - 1)
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bp_to_pickup++;
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else
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bp_to_pickup = buf;
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return free_bp;
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} else {
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/* No free blocks. Return NULL */
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return NULL;
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}
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}
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/*===========================================================================*
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* put_block *
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*===========================================================================*/
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void put_block(bp)
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register struct buf *bp; /* pointer to the buffer to be released */
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{
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if (bp == NULL) return; /* it is easier to check here than in caller */
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bp->b_count--; /* there is one use fewer now */
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}
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/*===========================================================================*
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* read_block *
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*===========================================================================*/
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static int read_block(bp)
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register struct buf *bp; /* buffer pointer */
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{
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int r;
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u64_t pos;
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vir_bytes block_size;
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block_size = v_pri.logical_block_size_l; /* The block size is indicated by
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* the superblock */
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pos = mul64u(bp->b_blocknr, block_size); /* get absolute position */
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r = bdev_read(fs_dev, pos, bp->b_data, block_size, BDEV_NOFLAGS);
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if (r != (ssize_t) block_size) {
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if (r == OK) r = END_OF_FILE;
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else printf("ISOFS(%d) I/O error on device %d/%d, block %u\n",
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SELF_E, (fs_dev>>MAJOR)&BYTE, (fs_dev>>MINOR)&BYTE,
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bp->b_blocknr);
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rdwt_err = r;
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return EINVAL;
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}
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return OK;
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}
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