Re: [PATCH] btrfs: relocation: output warning message for leftover v1 space cache before aborting current data balance

[Qu Wenruo <wqu@suse.com>]

On 2020/12/29 上午8:38, Qu Wenruo wrote:
> In delete_v1_space_cache(), if we find a leaf whose owner is tree root,
> and we can't grab the free space cache inode, then we return -ENOENT.
> 
> However this would make the caller, add_data_references(), to consider
> this as a critical error, and abort current data balance.
> 
> This happens for fs using free space cache v2, while still has v1 data
> left.
> 
> For v2 free space cache, we no longer load v1 data, making btrfs_igrab()
> no longer work for root tree to grab v1 free space cache inodes.
> 
> The proper fix for the problem is to delete v1 space cache completely
> during v2 convert.
> 
> We can't just ignore the -ENOENT error, as for root tree we don't use
> reloc tree to replace its data references, but rely on COW.
> This means, we have no way to relocate the leftover v1 data, and block
> the relocation.
> 
> This patch will just workaround it by outputting a warning message,
> showing the user how to manually solve it.
> 
> Reported-by: Stéphane Lesimple <stephane_btrfs2@lesimple.fr>
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Please discard this patch.

This didn't really solve the problem and may give the end user some 
false impression.

Thanks,
Qu
> ---
>   fs/btrfs/relocation.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
> 
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 19b7db8b2117..42746b59268d 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -3027,6 +3027,25 @@ int add_data_references(struct reloc_control *rc,
>   		ret = delete_v1_space_cache(eb, rc->block_group,
>   					    extent_key->objectid);
>   		free_extent_buffer(eb);
> +		/*
> +		 * This happens when the fs is converted to use v2 space cache,
> +		 * but some v1 data is still left.
> +		 * In that case, we can't delete the v1 space cache data as we
> +		 * can't grab the free space cache inode anymore.
> +		 *
> +		 * And we can't ignore the error, as for root tree (where v1
> +		 * space cache is) we don't do reloc tree to replace the data
> +		 * to the new location, thus the old data will still be there,
> +		 * blocking the data chunk to be relocated.
> +		 *
> +		 * Here we just warn user about the problem, and provide a
> +		 * workaround.
> +		 * The proper fix is in the v2 convert mount, which should
> +		 * completely remove v1 data.
> +		 */
> +		if (ret == -ENOENT)
> +			btrfs_warn(fs_info,
> +"leftover v1 space cache found, please use btrfs-check --clear-space-cache v1 to clean it up");
>   		if (ret < 0)
>   			break;
>   		ret = __add_tree_block(rc, ref_node->val, blocksize, blocks);
>