Re: [PATCH] ext4: fix ext4_init_block_bitmap() for metablock block group

Andreas Dilger
2008-06-16T05:19:04Z

On Jun 15, 2008 23:24 +0900, Akinobu Mita wrote:
> When meta_bg feature is enabled and s_first_meta_bg != 0,
> ext4_init_block_bitmap() miscalculates the number of block used by
> the group descriptor table (0 or 1 for metablock block group)

Can you please clarify why the calculation is incorrect? I admit that
I didn't test with META_BG enabled, so it could well be wrong, but looking
at the code I can't understand why it is incorrect.

> @@ -121,12 +121,7 @@ unsigned ext4_init_block_bitmap(struct s
if (!EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_META_BG) ||
block_group < le32_to_cpu(sbi->s_es->s_first_meta_bg) *
sbi->s_desc_per_block) {
if (bit_max) {
bit_max += ext4_bg_num_gdb(sb, block_group);
bit_max +=
> le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks);
> }
> } else { /* For META_BG_BLOCK_GROUPS */
> - int group_rel = (block_group -
> - le32_to_cpu(sbi->s_es->s_first_meta_bg)) %
> - EXT4_DESC_PER_BLOCK(sb);
> - if (group_rel == 0 || group_rel == 1 ||
> - (group_rel == EXT4_DESC_PER_BLOCK(sb) - 1))
> - bit_max += 1;
> + bit_max += ext4_bg_num_gdb(sb, block_group);
> }

As you can see, the "if" checks if the block group is before s_first_meta_bg
to treat it as a "normal" group, and only uses the "else" once beyond the
start of the s_first_meta_bg limit.

It definitely is less complex to use ext4_bg_num_gdb(), and this could
further be simplified by using ext4_bg_gdb_meta() in the "else" clause.
In fact, the whole if/else could be replaced with ext4_bg_num_gdb() if
it weren't for s_reserved_gdt_blocks.

Maybe it makes sense (cleaner code, less chance for bugs) to change the
ext4_bg_num_gdb() function to take a parameter on whether it should
include the s_reserved_gdt_blocks or not:

static unsigned long ext4_bg_num_gdb_nometa(struct super_block *sb, int group,
int reserved)
{
if (EXT4_HAS_RO_COMPAT_FEATURE(sb,
EXT4_FEATURE_RO_COMPAT_SPARSE_SUPER) &&
!ext4_group_sparse(group))
return 0;
return EXT4_SB(sb)->s_gdb_count +
reserved ? EXT4_SB(sb)->s_reserved_gdt_blocks : 0;
}

unsigned long ext4_bg_num_gdb(struct super_block *sb, int group, int reserved)
{
unsigned long first_meta_bg =
le32_to_cpu(EXT4_SB(sb)->s_es->s_first_meta_bg);
unsigned long metagroup = group / EXT4_DESC_PER_BLOCK(sb);

if (!EXT4_HAS_INCOMPAT_FEATURE(sb,EXT4_FEATURE_INCOMPAT_META_BG) ||
metagroup < first_meta_bg)
return ext4_bg_num_gdb_nometa(sb, group, reserved);

return ext4_bg_num_gdb_meta(sb, group);
}

The fewer places in the code that need to understand META_BG, the less
chance of having a bug. Now the code in ext4_init_block_bitmap() can be:

bit_max += ext4_bg_num_gdb(sb, block_group, 1);

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.