Re: [PATCH 10/13] btrfs-progs: check: fix bug in find_possible_backrefs

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2018/10/23 下午5:41, Su Yue wrote:
> From: Su Yanjun <suyj.fnst@cn.fujitsu.com>
> 
> It may cost more time to search all extent data of correspond files but
> should not influence total speed too much cause that only corrupted
> extent items are participated in.

Sorry, I didn't really get the point of the modification from the commit
message.

Would you please explain the purpose of this patch first?

Thanks,
Qu

> 
> Signed-off-by: Su Yanjun <suyj.fnst@cn.fujitsu.com>
> ---
>  check/main.c | 110 ++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 92 insertions(+), 18 deletions(-)
> 
> diff --git a/check/main.c b/check/main.c
> index bd1f322e0f12..90d9fd570287 100644
> --- a/check/main.c
> +++ b/check/main.c
> @@ -7015,6 +7015,89 @@ out:
>  	return ret ? ret : nr_del;
>  }
>  
> +/*
> + * Based extent backref item, we find all file extent items in the fs tree. By
> + * the info we can rebuild the extent backref item
> + */
> +static int __find_possible_backrefs(struct btrfs_root *root,
> +		u64 owner, u64 offset, u64 bytenr, u64 *refs_ret,
> +		u64 *bytes_ret)
> +{
> +	int ret = 0;
> +	struct btrfs_path path;
> +	struct btrfs_key key;
> +	struct btrfs_key found_key;
> +	struct btrfs_file_extent_item *fi;
> +	struct extent_buffer *leaf;
> +	u64 backref_offset, disk_bytenr;
> +	int slot;
> +
> +	btrfs_init_path(&path);
> +
> +	key.objectid = owner;
> +	key.type = BTRFS_INODE_ITEM_KEY;
> +	key.offset = 0;
> +
> +	ret = btrfs_search_slot(NULL, root, &key, &path, 0, 0);
> +	if (ret > 0)
> +		ret = -ENOENT;
> +	if (ret) {
> +		btrfs_release_path(&path);
> +		return ret;
> +	}
> +
> +	btrfs_release_path(&path);
> +
> +	key.objectid = owner;
> +	key.type = BTRFS_EXTENT_DATA_KEY;
> +	key.offset = 0;
> +
> +	ret = btrfs_search_slot(NULL, root, &key, &path, 0, 0);
> +	if (ret < 0) {
> +		btrfs_release_path(&path);
> +		return ret;
> +	}
> +
> +	while (1) {
> +		leaf = path.nodes[0];
> +		slot = path.slots[0];
> +
> +		if (slot >= btrfs_header_nritems(leaf)) {
> +			ret = btrfs_next_leaf(root, &path);
> +			if (ret) {
> +				if (ret > 0)
> +					ret = 0;
> +				break;
> +			}
> +
> +			leaf = path.nodes[0];
> +			slot = path.slots[0];
> +		}
> +
> +		btrfs_item_key_to_cpu(leaf, &found_key, slot);
> +		if ((found_key.objectid != owner) ||
> +			(found_key.type != BTRFS_EXTENT_DATA_KEY))
> +			break;
> +
> +		fi = btrfs_item_ptr(leaf, slot,
> +				struct btrfs_file_extent_item);
> +
> +		backref_offset = found_key.offset -
> +			btrfs_file_extent_offset(leaf, fi);
> +		disk_bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
> +		*bytes_ret = btrfs_file_extent_disk_num_bytes(leaf,
> +								fi);
> +		if ((disk_bytenr == bytenr) &&
> +			(backref_offset == offset)) {
> +			(*refs_ret)++;
> +		}
> +		path.slots[0]++;
> +	}
> +
> +	btrfs_release_path(&path);
> +	return ret;
> +}
> +
>  static int find_possible_backrefs(struct btrfs_fs_info *info,
>  				  struct btrfs_path *path,
>  				  struct cache_tree *extent_cache,
> @@ -7024,9 +7107,9 @@ static int find_possible_backrefs(struct btrfs_fs_info *info,
>  	struct extent_backref *back, *tmp;
>  	struct data_backref *dback;
>  	struct cache_extent *cache;
> -	struct btrfs_file_extent_item *fi;
>  	struct btrfs_key key;
>  	u64 bytenr, bytes;
> +	u64 refs;
>  	int ret;
>  
>  	rbtree_postorder_for_each_entry_safe(back, tmp,
> @@ -7054,24 +7137,15 @@ static int find_possible_backrefs(struct btrfs_fs_info *info,
>  		if (IS_ERR(root))
>  			return PTR_ERR(root);
>  
> -		key.objectid = dback->owner;
> -		key.type = BTRFS_EXTENT_DATA_KEY;
> -		key.offset = dback->offset;
> -		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> -		if (ret) {
> -			btrfs_release_path(path);
> -			if (ret < 0)
> -				return ret;
> -			/* Didn't find it, we can carry on */
> -			ret = 0;
> +		refs = 0;
> +		bytes = 0;
> +		ret = __find_possible_backrefs(root, dback->owner,
> +				dback->offset, rec->start, &refs, &bytes);
> +		if (ret)
>  			continue;
> -		}
>  
> -		fi = btrfs_item_ptr(path->nodes[0], path->slots[0],
> -				    struct btrfs_file_extent_item);
> -		bytenr = btrfs_file_extent_disk_bytenr(path->nodes[0], fi);
> -		bytes = btrfs_file_extent_disk_num_bytes(path->nodes[0], fi);
> -		btrfs_release_path(path);
> +		bytenr = rec->start;
> +
>  		cache = lookup_cache_extent(extent_cache, bytenr, 1);
>  		if (cache) {
>  			struct extent_record *tmp;
> @@ -7090,7 +7164,7 @@ static int find_possible_backrefs(struct btrfs_fs_info *info,
>  				continue;
>  		}
>  
> -		dback->found_ref += 1;
> +		dback->found_ref += refs;
>  		dback->disk_bytenr = bytenr;
>  		dback->bytes = bytes;
>  
>