Re: [PATCH v3] Btrfs-progs: optimize function btrfs_read_chunk_tree

[Miao Xie <miaox@cn.fujitsu.com>]

On 	tue, 30 Jul 2013 12:08:17 +0100, Filipe David Borba Manana wrote:
> After reading all device items from the chunk tree, don't
> exit the loop and then navigate down the tree again to find
> the chunk items. Instead just read all device items and
> chunk items with a single tree search. This is possible
> because all device items are found before any chunk item in
> the chunks tree.
> 
> This is a port of the corresponding kernel patch to keep both
> kernel and btrfs-progs identical:
> 
> https://patchwork.kernel.org/patch/2835529/
> 
> Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
> ---
> 
> V2: Simplified logic inside the loop
>     (suggested by Josef Bacik on irc).
> V3: Updated comment to comply with kernel coding style.

Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>

> 
>  volumes.c |   28 ++++++++++------------------
>  1 file changed, 10 insertions(+), 18 deletions(-)
> 
> diff --git a/volumes.c b/volumes.c
> index 0ff2283..0cb3505 100644
> --- a/volumes.c
> +++ b/volumes.c
> @@ -1718,14 +1718,15 @@ int btrfs_read_chunk_tree(struct btrfs_root *root)
>  	if (!path)
>  		return -ENOMEM;
>  
> -	/* first we search for all of the device items, and then we
> -	 * read in all of the chunk items.  This way we can create chunk
> -	 * mappings that reference all of the devices that are afound
> +	/*
> +	 * Read all device items, and then all the chunk items. All
> +	 * device items are found before any chunk item (their object id
> +	 * is smaller than the lowest possible object id for a chunk
> +	 * item - BTRFS_FIRST_CHUNK_TREE_OBJECTID).
>  	 */
>  	key.objectid = BTRFS_DEV_ITEMS_OBJECTID;
>  	key.offset = 0;
>  	key.type = 0;
> -again:
>  	ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
>  	while(1) {
>  		leaf = path->nodes[0];
> @@ -1739,16 +1740,12 @@ again:
>  			break;
>  		}
>  		btrfs_item_key_to_cpu(leaf, &found_key, slot);
> -		if (key.objectid == BTRFS_DEV_ITEMS_OBJECTID) {
> -			if (found_key.objectid != BTRFS_DEV_ITEMS_OBJECTID)
> -				break;
> -			if (found_key.type == BTRFS_DEV_ITEM_KEY) {
> -				struct btrfs_dev_item *dev_item;
> -				dev_item = btrfs_item_ptr(leaf, slot,
> +		if (found_key.type == BTRFS_DEV_ITEM_KEY) {
> +			struct btrfs_dev_item *dev_item;
> +			dev_item = btrfs_item_ptr(leaf, slot,
>  						  struct btrfs_dev_item);
> -				ret = read_one_dev(root, leaf, dev_item);
> -				BUG_ON(ret);
> -			}
> +			ret = read_one_dev(root, leaf, dev_item);
> +			BUG_ON(ret);
>  		} else if (found_key.type == BTRFS_CHUNK_ITEM_KEY) {
>  			struct btrfs_chunk *chunk;
>  			chunk = btrfs_item_ptr(leaf, slot, struct btrfs_chunk);
> @@ -1757,11 +1754,6 @@ again:
>  		}
>  		path->slots[0]++;
>  	}
> -	if (key.objectid == BTRFS_DEV_ITEMS_OBJECTID) {
> -		key.objectid = 0;
> -		btrfs_release_path(root, path);
> -		goto again;
> -	}
>  
>  	ret = 0;
>  error:
>