Re: [PATCH 1/2] btrfs: remove dead create_space_info calls

[Nikolay Borisov <nborisov@suse.com>]

On 20.03.2018 21:25, jeffm@suse.com wrote:
> From: Jeff Mahoney <jeffm@suse.com>
> 
> Since commit 2be12ef79 (btrfs: Separate space_info create/update), we've
> separated out the creation and updating of the space info structures.
> That commit was a straightforward refactoring of the two parts of
> update_space_info, but we can go a step further.  Since commits
> c59021f84 (Btrfs: fix OOPS of empty filesystem after balance) and
> b742bb82f (Btrfs: Link block groups of different raid types), we know
> that the space_info structures will be created at mount and there will
> only ever be, at most, three of them.
> 
> This patch cleans out the create_space_info calls after __find_space_info
> returns NULL since __find_space_info *can't* return NULL.
> 
> The initial cause for reviewing this was the kobject_add calls from
> create_space_info occuring in sites where fs-reclaim wasn't allowed.  Now
> we are certain they occur only early in the mount process and are safe.
> 
> Signed-off-by: Jeff Mahoney <jeffm@suse.com>

So when I did this cleanup I really wasn't sure under what conditions
space_info was allocated apart from mount time. I'm glad you were able
to prove it's not really done at any other time.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  fs/btrfs/ctree.h       |  6 +++---
>  fs/btrfs/extent-tree.c | 16 ++--------------
>  2 files changed, 5 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index da308774b8a4..ffbb05aa66fa 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -952,9 +952,9 @@ struct btrfs_fs_info {
>  	struct btrfs_fs_devices *fs_devices;
>  
>  	/*
> -	 * the space_info list is almost entirely read only.  It only changes
> -	 * when we add a new raid type to the FS, and that happens
> -	 * very rarely.  RCU is used to protect it.
> +	 * The space_info list is effectively read only after initial
> +	 * setup.  It is populated at mount time and cleaned up after
> +	 * all block groups are removed.  RCU is used to protect it.
>  	 */
>  	struct list_head space_info;
>  
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 2d5e963fae88..0e9a21230217 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4603,11 +4603,7 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
>  		return -ENOSPC;
>  
>  	space_info = __find_space_info(fs_info, flags);
> -	if (!space_info) {
> -		ret = create_space_info(fs_info, flags, &space_info);
> -		if (ret)
> -			return ret;
> -	}
> +	ASSERT(space_info);
>  
>  again:
>  	spin_lock(&space_info->lock);
> @@ -10255,15 +10251,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans,
>  	 * with its ->space_info set.
>  	 */
>  	cache->space_info = __find_space_info(fs_info, cache->flags);
> -	if (!cache->space_info) {
> -		ret = create_space_info(fs_info, cache->flags,
> -				       &cache->space_info);
> -		if (ret) {
> -			btrfs_remove_free_space_cache(cache);
> -			btrfs_put_block_group(cache);
> -			return ret;
> -		}
> -	}
> +	ASSERT(cache->space_info);
>  
>  	ret = btrfs_add_block_group_cache(fs_info, cache);
>  	if (ret) {
>