Re: [PATCH v2] ext4: Fix growing of tiny filesystems

[Eric Sandeen <sandeen@redhat.com>]

On 4/30/15 2:06 AM, Jan Kara wrote:
> The estimate of necessary transaction credits in ext4_flex_group_add()
> is too pessimistic. It reserves credit for sb, resize inode, and resize
> inode dindirect block for each group added in a flex group although they
> are always the same block and thus it is enough to account them only
> once. Also the number of modified GDT block is overestimated since we
> fit EXT4_DESC_PER_BLOCK(sb) descriptors in one block.
> 
> Make the estimation more precise. That reduces number of requested
> credits enough that we can grow 20 MB filesystem (which has 1 MB
> journal, 79 reserved GDT blocks, and flex group size 16 by default).
> 
> Signed-off-by: Jan Kara <jack@suse.cz>

Jan, thank you for looking into this and sorting it out.

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

> ---
>  fs/ext4/resize.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
> index 8a8ec6293b19..cf0c472047e3 100644
> --- a/fs/ext4/resize.c
> +++ b/fs/ext4/resize.c
> @@ -1432,12 +1432,15 @@ static int ext4_flex_group_add(struct super_block *sb,
>  		goto exit;
>  	/*
>  	 * We will always be modifying at least the superblock and  GDT
> -	 * block.  If we are adding a group past the last current GDT block,
> +	 * blocks.  If we are adding a group past the last current GDT block,
>  	 * we will also modify the inode and the dindirect block.  If we
>  	 * are adding a group with superblock/GDT backups  we will also
>  	 * modify each of the reserved GDT dindirect blocks.
>  	 */
> -	credit = flex_gd->count * 4 + reserved_gdb;
> +	credit = 3;	/* sb, resize inode, resize inode dindirect */
> +	/* GDT blocks */
> +	credit += 1 + DIV_ROUND_UP(flex_gd->count, EXT4_DESC_PER_BLOCK(sb));
> +	credit += reserved_gdb;	/* Reserved GDT dindirect blocks */
>  	handle = ext4_journal_start_sb(sb, EXT4_HT_RESIZE, credit);
>  	if (IS_ERR(handle)) {
>  		err = PTR_ERR(handle);
>