Re: [PATCH v3] btrfs: rescue: allow ibadroots to skip bad extent tree when reading block group items

[Anand Jain <anand.jain@oracle.com>]

On 15/07/2021 13:00, Qu Wenruo wrote:
> When extent tree gets corrupted, normally it's not extent tree root, but
> one toasted tree leaf/node.
> 
> In that case, rescue=ibadroots mount option won't help as it can only
> handle the extent tree root corruption.
> 
> This patch will enhance the behavior by:
> 
> - Allow fill_dummy_bgs() to ignore -EEXIST error
> 
>    This means we may have some block group items read from disk, but
>    then hit some error halfway.
> 
> - Fallback to fill_dummy_bgs() if any error gets hit in
>    btrfs_read_block_groups()
> 
>    Of course, this still needs rescue=ibadroots mount option.
> 
> With that, rescue=ibadroots can handle extent tree corruption more
> gracefully and allow a better recover chance.
> 
> Reported-by: Zhenyu Wu <wuzy001@gmail.com>
> Link: https://www.spinics.net/lists/linux-btrfs/msg114424.html
> Signed-off-by: Qu Wenruo <wqu@suse.com>

  Reviewed-by: Anand Jain <anand.jain@oracle.com>

Thanks, Anand

> ---
> Changelog:
> v2:
> - Don't try to fill with dummy block groups when we hit ENOMEM
> v3:
> - Remove a dead condition
>    The empty fs_info->extent_root case has already been handled.
> ---
>   fs/btrfs/block-group.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 5bd76a45037e..9bc68515bc4a 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -2105,11 +2105,16 @@ static int fill_dummy_bgs(struct btrfs_fs_info *fs_info)
>   		bg->used = em->len;
>   		bg->flags = map->type;
>   		ret = btrfs_add_block_group_cache(fs_info, bg);
> -		if (ret) {
> +		/*
> +		 * We may have some block groups filled already, thus ignore
> +		 * the -EEXIST error.
> +		 */
> +		if (ret && ret != -EEXIST) {
>   			btrfs_remove_free_space_cache(bg);
>   			btrfs_put_block_group(bg);
>   			break;
>   		}
> +		ret = 0;
>   		btrfs_update_space_info(fs_info, bg->flags, em->len, em->len,
>   					0, 0, &space_info);
>   		bg->space_info = space_info;
> @@ -2212,6 +2217,14 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
>   	ret = check_chunk_block_group_mappings(info);
>   error:
>   	btrfs_free_path(path);
> +	/*
> +	 * We hit some error reading the extent tree, and have rescue=ibadroots
> +	 * mount option.
> +	 * Try to fill using dummy block groups so that the user can continue
> +	 * to mount and grab their data.
> +	 */
> +	if (ret && btrfs_test_opt(info, IGNOREBADROOTS))
> +		ret = fill_dummy_bgs(info);
>   	return ret;
>   }
>   
>