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Date:	Mon, 16 Jun 2008 23:10:17 +0900
From:	Akinobu Mita <akinobu.mita@...il.com>
To:	Andreas Dilger <adilger@....com>
Cc:	linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Tweedie <sct@...hat.com>, adilger@...sterfs.com,
	Mingming Cao <cmm@...ibm.com>, Theodore Tso <tytso@....EDU>
Subject: Re: [PATCH] ext4: fix ext4_init_block_bitmap() for metablock block
	group

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@...il.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@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Stephen Tweedie <sct@...hat.com>
Cc: adilger@...sterfs.com
Cc: Mingming Cao <cmm@...ibm.com>
Cc: Theodore Tso <tytso@....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;
--
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