Re: [PATCH v2 1/3] btrfs: backref: Introduce the skeleton of btrfs_backref_iterator

[Nikolay Borisov <nborisov@suse.com>]

On 14.02.20 г. 10:13 ч., Qu Wenruo wrote:
> Due to the complex nature of btrfs extent tree, when we want to iterate
> all backrefs of one extent, it involves quite a lot of works, like
> search the EXTENT_ITEM/METADATA_ITEM, iteration through inline and keyed
> backrefs.
> 
> Normally this would result pretty complex code, something like:
>   btrfs_search_slot()
>   /* Ensure we are at EXTENT_ITEM/METADATA_ITEM */
>   while (1) {	/* Loop for extent tree items */
> 	while (ptr < end) { /* Loop for inlined items */
> 		/* REAL WORK HERE */
> 	}
>   next:
>   	ret = btrfs_next_item()
> 	/* Ensure we're still at keyed item for specified bytenr */
>   }
> 
> The idea of btrfs_backref_iterator is to avoid such complex and hard to
> read code structure, but something like the following:
> 
>   iterator = btrfs_backref_iterator_alloc();
>   ret = btrfs_backref_iterator_start(iterator, bytenr);
>   if (ret < 0)
> 	goto out;
>   for (; ; ret = btrfs_backref_iterator_next(iterator)) {
> 	/* REAL WORK HERE */
>   }
>   out:
>   btrfs_backref_iterator_free(iterator);
> 
> This patch is just the skeleton + btrfs_backref_iterator_start() code.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/backref.c | 58 +++++++++++++++++++++++++++++++++++
>  fs/btrfs/backref.h | 76 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 134 insertions(+)
> 
> diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
> index e5d85311d5d5..73c4829609c9 100644
> --- a/fs/btrfs/backref.c
> +++ b/fs/btrfs/backref.c
> @@ -2252,3 +2252,61 @@ void free_ipath(struct inode_fs_paths *ipath)
>  	kvfree(ipath->fspath);
>  	kfree(ipath);
>  }
> +
> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
> +				 u64 bytenr)
> +{
> +	struct btrfs_fs_info *fs_info = iterator->fs_info;
> +	struct btrfs_path *path = iterator->path;
> +	struct btrfs_extent_item *ei;
> +	struct btrfs_key key;
> +	int ret;
> +
> +	key.objectid = bytenr;
> +	key.type = BTRFS_METADATA_ITEM_KEY;
> +	key.offset = (u64)-1;
> +
> +	ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
> +	if (ret < 0)
> +		return ret;
> +	if (ret == 0) {
> +		ret = -EUCLEAN;
> +		goto release;
> +	}
> +	if (path->slots[0] == 0) {
> +		WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
> +		ret = -EUCLEAN;
> +		goto release;
> +	}
> +	path->slots[0]--;
> +
> +	btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> +	if (!(key.type == BTRFS_EXTENT_ITEM_KEY ||
> +	      key.type == BTRFS_METADATA_ITEM_KEY) || key.objectid != bytenr) {
> +		ret = -ENOENT;
> +		goto release;
> +	}
> +	memcpy(&iterator->cur_key, &key, sizeof(key));
> +	iterator->end_ptr = btrfs_item_end_nr(path->nodes[0], path->slots[0]);
> +	iterator->item_ptr = btrfs_item_ptr_offset(path->nodes[0],
> +						   path->slots[0]);
> +	ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
> +			    struct btrfs_extent_item);
> +
> +	/* Only support iteration on tree backref yet */
> +	if (btrfs_extent_flags(path->nodes[0], ei) & BTRFS_EXTENT_FLAG_DATA) {
> +		ret = -ENOTTY;
> +		goto release;
> +	}

Isn't this implied bye the fact you are searching for METADATA ITEMS to
begin with ? Considering this shouldn't detecting EXTENT_FLAG_DATA in
the backrefs of a METADATA_EXTENT be considered a corruption?

> +	iterator->cur_ptr = iterator->item_ptr + sizeof(*ei);
> +	return 0;
> +release:
> +	iterator->end_ptr = 0;
> +	iterator->cur_ptr = 0;
> +	iterator->item_ptr = 0;
> +	iterator->cur_key.objectid = 0;
> +	iterator->cur_key.type = 0;
> +	iterator->cur_key.offset = 0;
> +	btrfs_release_path(path);
> +	return ret;
> +}
> diff --git a/fs/btrfs/backref.h b/fs/btrfs/backref.h
> index 777f61dc081e..b53301f2147f 100644
> --- a/fs/btrfs/backref.h
> +++ b/fs/btrfs/backref.h
> @@ -74,4 +74,80 @@ struct prelim_ref {
>  	u64 wanted_disk_byte;
>  };
>  
> +/*
> + * Helper structure to help iterate backrefs of one extent.
> + *
> + * Now it only supports iteration for tree block in commit root.
> + */
> +struct btrfs_backref_iterator {
> +	u64 bytenr;
> +	struct btrfs_path *path;
> +	struct btrfs_fs_info *fs_info;
> +	struct btrfs_key cur_key;
> +	unsigned long item_ptr;
> +	unsigned long cur_ptr;
> +	unsigned long end_ptr;
> +};
> +
> +static inline struct btrfs_backref_iterator *
> +btrfs_backref_iterator_alloc(struct btrfs_fs_info *fs_info, gfp_t gfp_flag)
> +{
> +	struct btrfs_backref_iterator *ret;
> +
> +	ret = kzalloc(sizeof(*ret), gfp_flag);
> +	if (!ret)
> +		return NULL;
> +
> +	ret->path = btrfs_alloc_path();
> +	if (!ret) {
> +		kfree(ret);
> +		return NULL;
> +	}
> +
> +	/* Current backref iterator only supports iteration in commit root */
> +	ret->path->search_commit_root = 1;
> +	ret->path->skip_locking = 1;
> +	ret->path->reada = READA_FORWARD;
> +	ret->fs_info = fs_info;
> +
> +	return ret;
> +}
> +
> +static inline void btrfs_backref_iterator_free(
> +		struct btrfs_backref_iterator *iterator)
> +{
> +	if (!iterator)
> +		return;
> +	btrfs_free_path(iterator->path);
> +	kfree(iterator);
> +}
> +
> +static inline struct extent_buffer *
> +btrfs_backref_get_eb(struct btrfs_backref_iterator *iterator)
> +{
> +	if (!iterator)
> +		return NULL;
> +	return iterator->path->nodes[0];
> +}
> +
> +/*
> + * For metadata with EXTENT_ITEM key (non-skinny) case, the first inline data
> + * is btrfs_tree_block_info, without a btrfs_extent_inline_ref header.
> + *
> + * This helper is here to determine if that's the case.
> + */
> +static inline bool btrfs_backref_has_tree_block_info(
> +		struct btrfs_backref_iterator *iterator)
> +{
> +	if (iterator->cur_key.type == BTRFS_EXTENT_ITEM_KEY &&
> +	    iterator->cur_ptr - iterator->item_ptr ==
> +	    sizeof(struct btrfs_extent_item))
> +		return true;
> +	return false;
> +}
> +
> +int btrfs_backref_iterator_start(struct btrfs_backref_iterator *iterator,
> +				 u64 bytenr);
> +int btrfs_backref_iterator_next(struct btrfs_backref_iterator *iterator);
> +
>  #endif
>