Re: [PATCH] Btrfs: eliminate the exceptional root_tree refs=0

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

On 	thu, 5 Sep 2013 16:58:43 +0200, Stefan Behrens wrote:
> The fact that btrfs_root_refs() returned 0 for the tree_root caused
> bugs in the past, therefore it is set to 1 with this patch and
> (hopefully) all affected code is adapted to this change.
> 
> I verified this change by temporarily adding WARN_ON() checks
> everywhere where btrfs_root_refs() is used, checking whether the
> logic of the code is changed by btrfs_root_refs() returning 1
> instead of 0 for root->root_key.objectid == BTRFS_ROOT_TREE_OBJECTID.
> With these added checks, I ran the xfstests './check -g auto'.
> 
> The two roots chunk_root and log_root_tree that are only referenced
> by the superblock and the log_roots below the log_root_tree still
> have btrfs_root_refs() == 0, only the tree_root is changed.
> 
> Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
> ---
>  fs/btrfs/disk-io.c   |  1 +
>  fs/btrfs/inode-map.c |  3 +--
>  fs/btrfs/inode.c     | 21 ++++++++-------------
>  3 files changed, 10 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index fa8b2c6..ffc3e43 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2667,6 +2667,7 @@ retry_root_backup:
>  
>  	btrfs_set_root_node(&tree_root->root_item, tree_root->node);
>  	tree_root->commit_root = btrfs_root_node(tree_root);
> +	btrfs_set_root_refs(&tree_root->root_item, 1);
>  
>  	location.objectid = BTRFS_EXTENT_TREE_OBJECTID;
>  	location.type = BTRFS_ROOT_ITEM_KEY;
> diff --git a/fs/btrfs/inode-map.c b/fs/btrfs/inode-map.c
> index 2c66ddb..d11e1c6 100644
> --- a/fs/btrfs/inode-map.c
> +++ b/fs/btrfs/inode-map.c
> @@ -412,8 +412,7 @@ int btrfs_save_ino_cache(struct btrfs_root *root,
>  		return 0;
>  
>  	/* Don't save inode cache if we are deleting this root */
> -	if (btrfs_root_refs(&root->root_item) == 0 &&
> -	    root != root->fs_info->tree_root)
> +	if (btrfs_root_refs(&root->root_item) == 0)
>  		return 0;
>  
>  	if (!btrfs_test_opt(root, INODE_MAP_CACHE))
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 26992ee..7d0ef55 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -4472,8 +4472,10 @@ void btrfs_evict_inode(struct inode *inode)
>  	trace_btrfs_inode_evict(inode);
>  
>  	truncate_inode_pages(&inode->i_data, 0);
> -	if (inode->i_nlink && (btrfs_root_refs(&root->root_item) != 0 ||
> -			       btrfs_is_free_space_inode(inode)))
> +	if (inode->i_nlink &&
> +	    ((btrfs_root_refs(&root->root_item) != 0 &&
> +	      root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID) ||
> +	     btrfs_is_free_space_inode(inode)))

if (inode->i_nlink && btrfs_root_refs(&root->root_item)) {
}

is OK, because with this patch, the refs of the free space inodes' root(tree root)
should be 1. For the ino cache inodes' root(fs/file root), if the root is dead,
we can drop the ino cache inode safely. And if the root is not dead, the refs
should be > 0, we will skip the drop.

>  		goto no_delete;
>  
>  	if (is_bad_inode(inode)) {
> @@ -4490,7 +4492,8 @@ void btrfs_evict_inode(struct inode *inode)
>  	}
>  
>  	if (inode->i_nlink > 0) {
> -		BUG_ON(btrfs_root_refs(&root->root_item) != 0);
> +		BUG_ON(btrfs_root_refs(&root->root_item) != 0 &&
> +		       root->root_key.objectid != BTRFS_ROOT_TREE_OBJECTID);

This change is unnecessary because the refs of the root tree is 1 now.

>  		goto no_delete;
>  	}
>  
> @@ -4731,14 +4734,7 @@ static void inode_tree_del(struct inode *inode)
>  	}
>  	spin_unlock(&root->inode_lock);
>  
> -	/*
> -	 * Free space cache has inodes in the tree root, but the tree root has a
> -	 * root_refs of 0, so this could end up dropping the tree root as a
> -	 * snapshot, so we need the extra !root->fs_info->tree_root check to
> -	 * make sure we don't drop it.
> -	 */
> -	if (empty && btrfs_root_refs(&root->root_item) == 0 &&
> -	    root != root->fs_info->tree_root) {
> +	if (empty && btrfs_root_refs(&root->root_item) == 0) {
>  		synchronize_srcu(&root->fs_info->subvol_srcu);
>  		spin_lock(&root->inode_lock);
>  		empty = RB_EMPTY_ROOT(&root->inode_tree);
> @@ -7872,8 +7868,7 @@ int btrfs_drop_inode(struct inode *inode)
>  		return 1;
>  
>  	/* the snap/subvol tree is on deleting */
> -	if (btrfs_root_refs(&root->root_item) == 0 &&
> -	    root != root->fs_info->tree_root)
> +	if (btrfs_root_refs(&root->root_item) == 0)
>  		return 1;
>  	else
>  		return generic_drop_inode(inode);

The others is OK.

Thanks
Miao