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Date:	Mon, 20 Feb 2012 18:20:29 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	Milan Broz <mbroz@...hat.com>
Subject: Re: [PATCH] ext4: fix resize when resizing within single group

On 2/2/12 3:05 AM, Lukas Czerner wrote:
> When resizing file system in the way that the new size of the file
> system is still in the same group (no new groups are added), then we can
> hit a BUG_ON in ext4_alloc_group_tables()
> 
> BUG_ON(flex_gd->count == 0 || group_data == NULL);
> 
> because flex_gd->count is zero. The reason is the missing check for such
> case, so the code always extend the last group fully and then attempt to
> add more groups, but at that time n_blocks_count is actually smaller
> than o_blocks_count.
> 
> It can be easily reproduced like this:
> 
> mkfs.ext4 -b 4096 /dev/sda 30M
> mount /dev/sda /mnt/test
> resize2fs /dev/sda 50M
> 
> Fix this by checking whether the resize happens within the singe group
> and only add that many blocks into the last group to satisfy user
> request. Then o_blocks_count == n_blocks_count and the resize will exit
> successfully without and attempt to add more groups into the fs.
> 
> Also fix mixing together block number and blocks count which might be
> confusing and can easily lead to off-by-one errors (but it is actually
> not the case here since the two occurrence of this mix-up will cancel
> each other).
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> Reported-by: Milan Broz <mbroz@...hat.com>

Reviewed-by: Eric Sandeen <sandeen@...hat.com>

Ted, if this hits linux-next we can pull it into fedora and close a bug they're tracking.  :)

thanks,
-Eric

> ---
>  fs/ext4/resize.c |   14 ++++++++------
>  1 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index f9d948f..3fed79d 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -1582,7 +1582,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
>  	ext4_fsblk_t o_blocks_count;
>  	ext4_group_t o_group;
>  	ext4_group_t n_group;
> -	ext4_grpblk_t offset;
> +	ext4_grpblk_t offset, add;
>  	unsigned long n_desc_blocks;
>  	unsigned long o_desc_blocks;
>  	unsigned long desc_blocks;
> @@ -1605,7 +1605,7 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
>  		return 0;
>  
>  	ext4_get_group_no_and_offset(sb, n_blocks_count - 1, &n_group, &offset);
> -	ext4_get_group_no_and_offset(sb, o_blocks_count, &o_group, &offset);
> +	ext4_get_group_no_and_offset(sb, o_blocks_count - 1, &o_group, &offset);
>  
>  	n_desc_blocks = (n_group + EXT4_DESC_PER_BLOCK(sb)) /
>  			EXT4_DESC_PER_BLOCK(sb);
> @@ -1634,10 +1634,12 @@ int ext4_resize_fs(struct super_block *sb, ext4_fsblk_t n_blocks_count)
>  	}
>  	brelse(bh);
>  
> -	if (offset != 0) {
> -		/* extend the last group */
> -		ext4_grpblk_t add;
> -		add = EXT4_BLOCKS_PER_GROUP(sb) - offset;
> +	/* extend the last group */
> +	if (n_group == o_group)
> +		add = n_blocks_count - o_blocks_count;
> +	else
> +		add = EXT4_BLOCKS_PER_GROUP(sb) - (offset + 1);
> +	if (add > 0) {
>  		err = ext4_group_extend_no_check(sb, o_blocks_count, add);
>  		if (err)
>  			goto out;

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