Re: [PATCH v2] fs/minix: Verify bitmap block counts before mounting

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Aug 19, 2011 at 02:02:00PM -0400, Josh Boyer wrote:

> -unsigned long minix_count_free_blocks(struct minix_sb_info *sbi)
> +unsigned long minix_count_free_blocks(struct super_block *sb)
>  {
> -	return (count_free(sbi->s_zmap, sbi->s_zmap_blocks,
> -		sbi->s_nzones - sbi->s_firstdatazone + 1)
> +	struct minix_sb_info *sbi = minix_sb(sb);
> +	u32 bits = sbi->s_nzones - (sbi->s_firstdatazone + 1);
> +	unsigned blocks = minix_blocks_needed(bits, sb->s_blocksize);
> +
> +	return (count_free(sbi->s_zmap, blocks, bits)
>  		<< sbi->s_log_zone_size);
>  }

> +	unsigned blocks = minix_blocks_needed(bits, sb->s_blocksize);
> +
> +	return count_free(sbi->s_imap, blocks, bits);

I'd pass sb->s_blocksize to that sucker instead of blocks; less redundancy
and we have good uses for it there anyway.

> +static inline unsigned minix_blocks_needed(unsigned bits, unsigned blocksize)
> +{
> +	unsigned blocks = bits / (blocksize * 8);
> +
> +	if (bits % (blocksize * 8))
> +		blocks++;
> +	return blocks;
> +}

Yecchh...  The usual idiom for that is DIV_ROUND_UP(bits, blocksize * 8)
(see linux/kernel.h for details).