Re: [PATCH 1/7] fs: btrfs: Introduce btrfs_for_each_slot

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 24/08/2021 19:13, Marcos Paulo de Souza wrote:
> There is a common pattern when search for a key in btrfs:
> 
>  * Call btrfs_search_slot
>  * Endless loop
> 	 * If the found slot is bigger than the current items in the leaf, check the
> 	   next one
> 	 * If still not found in the next leaf, return 1
> 	 * Do something with the code
> 	 * Increment current slot, and continue
> 
> This pattern can be improved by creating an iterator macro, similar to
> those for_each_X already existing in the linux kernel. Using this
> approach means to reduce significantly boilerplate code, along making it
> easier to newcomers to understand how to code works.
> 
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
> ---
>  fs/btrfs/ctree.c | 28 ++++++++++++++++++++++++++++
>  fs/btrfs/ctree.h | 25 +++++++++++++++++++++++++
>  fs/btrfs/xattr.c | 40 ++++++++++++----------------------------
>  3 files changed, 65 insertions(+), 28 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
> index 84627cbd5b5b..b1aa6e3987d0 100644
> --- a/fs/btrfs/ctree.c
> +++ b/fs/btrfs/ctree.c
> @@ -2122,6 +2122,34 @@ int btrfs_search_backwards(struct btrfs_root *root, struct btrfs_key *key,
>  	return ret;
>  }
>  
> +/* Search for a valid slot for the given path.
> + * @root: The root node of the tree.
> + * @key: Will contain a valid item if found.
> + * @path: The start point to validate the slot.
> + *
> + * Return 0 if the item is valid, 1 if not found and < 0 if error.
> + */
> +int btrfs_valid_slot(struct btrfs_root *root, struct btrfs_key *key,
> +		     struct btrfs_path *path)
> +{
> +	while (1) {
> +		int ret;
> +		const int slot = path->slots[0];
> +		const struct extent_buffer *leaf = path->nodes[0];
> +
> +		if (slot >= btrfs_header_nritems(leaf)) {
> +			ret = btrfs_next_leaf(root, path);
> +			if (ret)
> +				return ret;
> +			continue;
> +		}
> +		btrfs_item_key_to_cpu(leaf, key, slot);
> +		break;
> +	}
> +
> +	return 0;
> +}
> +
>  /*
>   * adjust the pointers going up the tree, starting at level
>   * making sure the right key of each node is points to 'key'.
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index f07c82fafa04..1e3c4a7741ca 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -2912,6 +2912,31 @@ int btrfs_next_old_leaf(struct btrfs_root *root, struct btrfs_path *path,
>  int btrfs_search_backwards(struct btrfs_root *root, struct btrfs_key *key,
>  			   struct btrfs_path *path);
>  
> +int btrfs_valid_slot(struct btrfs_root *root, struct btrfs_key *key,
> +		     struct btrfs_path *path);
> +
> +/* Search in @root for a given @key, and store the slot found in @found_key.
> + * @root: The root node of the tree.
> + * @key: The key we are looking for.
> + * @found_key: Will hold the found item.
> + * @path: Holds the current slot/leaf.
> + * @iter_ret: Contains the returned value from btrfs_search_slot and
> + *            btrfs_valid_slot, whatever is executed later.
> + *
> + * The iter_ret is an output variable that will contain the result of the
> + * btrfs_search_slot if it returns an error, or the value returned from
> + * btrfs_valid_slot otherwise. The return value can be 0 if the something was
> + * found, 1 if there weren't bigger leaves, and <0 if error.
> + */
> +#define btrfs_for_each_slot(root, key, found_key, path, iter_ret)		\
> +	for (iter_ret = btrfs_search_slot(NULL, root, key, path, 0, 0);		\
> +		(								\
> +			iter_ret >= 0 &&					\
> +			(iter_ret = btrfs_valid_slot(root, found_key, path)) == 0 \
> +		);								  \
> +		path->slots[0]++						  \
> +	)
> +
>  static inline int btrfs_next_old_item(struct btrfs_root *root,
>  				      struct btrfs_path *p, u64 time_seq)
>  {

Shouldn't below xattr code be in a separate patch? Just like the block-group,
dev-replace, etc changes?

> diff --git a/fs/btrfs/xattr.c b/fs/btrfs/xattr.c
> index 8a4514283a4b..f85febba1891 100644
> --- a/fs/btrfs/xattr.c
> +++ b/fs/btrfs/xattr.c
> @@ -274,10 +274,12 @@ int btrfs_setxattr_trans(struct inode *inode, const char *name,
>  ssize_t btrfs_listxattr(struct dentry *dentry, char *buffer, size_t size)
>  {
>  	struct btrfs_key key;
> +	struct btrfs_key found_key;
>  	struct inode *inode = d_inode(dentry);
>  	struct btrfs_root *root = BTRFS_I(inode)->root;
>  	struct btrfs_path *path;
>  	int ret = 0;
> +	int iter_ret = 0;
>  	size_t total_size = 0, size_left = size;
>  
>  	/*
> @@ -295,44 +297,22 @@ ssize_t btrfs_listxattr(struct dentry *dentry, char *buffer, size_t size)
>  	path->reada = READA_FORWARD;
>  
>  	/* search for our xattrs */
> -	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> -	if (ret < 0)
> -		goto err;
> -
> -	while (1) {
> +	btrfs_for_each_slot(root, &key, &found_key, path, iter_ret) {
>  		struct extent_buffer *leaf;
>  		int slot;
>  		struct btrfs_dir_item *di;
> -		struct btrfs_key found_key;
>  		u32 item_size;
>  		u32 cur;
>  
>  		leaf = path->nodes[0];
>  		slot = path->slots[0];
>  
> -		/* this is where we start walking through the path */
> -		if (slot >= btrfs_header_nritems(leaf)) {
> -			/*
> -			 * if we've reached the last slot in this leaf we need
> -			 * to go to the next leaf and reset everything
> -			 */
> -			ret = btrfs_next_leaf(root, path);
> -			if (ret < 0)
> -				goto err;
> -			else if (ret > 0)
> -				break;
> -			continue;
> -		}
> -
> -		btrfs_item_key_to_cpu(leaf, &found_key, slot);
> -
>  		/* check to make sure this item is what we want */
> -		if (found_key.objectid != key.objectid)
> -			break;
> -		if (found_key.type > BTRFS_XATTR_ITEM_KEY)
> +		if (found_key.objectid != key.objectid ||
> +		    found_key.type > BTRFS_XATTR_ITEM_KEY)
>  			break;
>  		if (found_key.type < BTRFS_XATTR_ITEM_KEY)
> -			goto next_item;
> +			continue;
>  
>  		di = btrfs_item_ptr(leaf, slot, struct btrfs_dir_item);
>  		item_size = btrfs_item_size_nr(leaf, slot);
> @@ -365,9 +345,13 @@ ssize_t btrfs_listxattr(struct dentry *dentry, char *buffer, size_t size)
>  			cur += this_len;
>  			di = (struct btrfs_dir_item *)((char *)di + this_len);
>  		}
> -next_item:
> -		path->slots[0]++;
>  	}
> +
> +	if (iter_ret < 0) {
> +		ret = iter_ret;
> +		goto err;
> +	}
> +
>  	ret = total_size;
>  
>  err:
>