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

Akinobu Mita
2008-06-16T14:11:41Z

On Sun, Jun 15, 2008 at 11:19:00PM -0600, Andreas Dilger wrote:
> 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.

group_rel in the original code should be calculated by:

group_rel = block_group % EXT4_DESC_PER_BLOCK(sb);

No need to subtract s_first_meta_bg from brock_group.
It will be equivalent what ext4_bg_num_gdb_meta() does

> > @@ -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);

Looks good to me. But I'd like to divide into the change into simple
bugfix and cleanup patch in order to clarify the each change.

Here is a bugfix only patch.

From: Akinobu Mita <akinobu.mita@gmail.com>
Subject: ext4: fix ext4_init_block_bitmap() for metablock block group

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)

group_rel in the original code should be calculated by:

group_rel = block_group % EXT4_DESC_PER_BLOCK(sb);

No need to subtract s_first_meta_bg from brock_group.
It will be equivalent what ext4_bg_num_gdb_meta() does

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: adilger@clusterfs.com
Cc: Mingming Cao <cmm@us.ibm.com>
Cc: Theodore Tso <tytso@MIT.EDU>
---
fs/ext4/balloc.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

Index: 2.6-git/fs/ext4/balloc.c
===================================================================
--- 2.6-git.orig/fs/ext4/balloc.c
+++ 2.6-git/fs/ext4/balloc.c
@@ -121,9 +121,8 @@ unsigned ext4_init_block_bitmap(struct s
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);
+ int group_rel = block_group % EXT4_DESC_PER_BLOCK(sb);
+
if (group_rel == 0 || group_rel == 1 ||
(group_rel == EXT4_DESC_PER_BLOCK(sb) - 1))
bit_max += 1;