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Block Allocator
Xv6’s block allocator maintains a free bitmap on disk, with on bit per block.
Find the first available block, mark as in-use.
#define BSIZE 1024 // block size
// Block containing inode I
#define IBLOCK(I, sb) ((i) / IPB + sb.inodestart)
// Bitmap bits per block
#define BPB (BSIZE*8)
// Block of free map containing bit for block b
#define BBLOCK(b, sb) ((b)/BPB + sb.bmapstart)
Note:
sb is superblock. struct superblock sb;static uint
balloc(uint dev)
{
int b, bi, m;
struct buf *bp;
bp = 0;
for(b = 0; b < sb.size; b += BPB){
bp = bread(dev, BBLOCK(b, sb));
for(bi = 0; bi < BPB && b + bi < sb.size; bi++){
m = 1 << (bi % 8);
if((bp->data[bi/8] & m) == 0){ // Is block free?
bp->data[bi/8] |= m; // Mark block in use.
log_write(bp);
brelse(bp);
bzero(dev, b + bi);
return b + bi;
}
}
brelse(bp);
}
panic("balloc: out of blocks");
}
The outer loop reads each block of bitmap bits. The inner loop checks all
BPB bits in a single bitmap block.Find it and mark the bit as 0.
static void
bfree(int dev, uint b)
{
struct buf *bp;
int bi, m;
bp = bread(dev, BBLOCK(b, sb));
bi = b % BPB;
m = 1 << (bi % 8);
if((bp->data[bi/8] & m) == 0)
panic(“freeing free block”);
bp->data[bi/8] &= ~m;
log_write(bp);
brelse(bp);
}