Re: [PATCH v2 2/2] btrfs: zoned: activate block group only for extent allocation

[David Sterba <dsterba@suse.cz>]

On Tue, Mar 22, 2022 at 06:11:34PM +0900, Naohiro Aota wrote:
> In btrfs_make_block_group(), we activate the allocated block group,
> expecting that the block group is soon used for allocation. However, the
> chunk allocation from flush_space() context broke the assumption. There can
> be a large time gap between the chunk allocation time and the extent
> allocation time from the chunk.
> 
> Activating the empty block groups pre-allocated from flush_space() context
> can exhaust the active zone counter of a device. Once we use all the active
> zone counts for empty pre-allocated BGs, we cannot activate new BG for the
> other things: metadata, tree-log, or data relocation BG. That failure
> results in a fake -ENOSPC.
> 
> This patch introduces CHUNK_ALLOC_FORCE_FOR_EXTENT to distinguish the chunk
> allocation from find_free_extent(). Now, the new block group is activated
> only in that context.
> 
> Fixes: eb66a010d518 ("btrfs: zoned: activate new block group")
> Cc: stable@vger.kernel.org # 5.16+: c7d1b9109dd0: btrfs: return allocated block group from do_chunk_alloc()
> Cc: stable@vger.kernel.org # 5.16+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>  fs/btrfs/block-group.c | 24 ++++++++++++++++--------
>  fs/btrfs/block-group.h |  1 +
>  fs/btrfs/extent-tree.c |  2 +-
>  3 files changed, 18 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index d4ac1c76f539..84c97d76de92 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -2481,12 +2481,6 @@ struct btrfs_block_group *btrfs_make_block_group(struct btrfs_trans_handle *tran
>  		return ERR_PTR(ret);
>  	}
>  
> -	/*
> -	 * New block group is likely to be used soon. Try to activate it now.
> -	 * Failure is OK for now.
> -	 */
> -	btrfs_zone_activate(cache);
> -
>  	ret = exclude_super_stripes(cache);
>  	if (ret) {
>  		/* We may have excluded something, so call this just in case */
> @@ -3642,8 +3636,14 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags,
>  	struct btrfs_block_group *ret_bg;
>  	bool wait_for_alloc = false;
>  	bool should_alloc = false;
> +	bool from_extent_allocation = false;
>  	int ret = 0;
>  
> +	if (force == CHUNK_ALLOC_FORCE_FOR_EXTENT) {
> +		from_extent_allocation = true;
> +		force = CHUNK_ALLOC_FORCE;
> +	}
> +
>  	/* Don't re-enter if we're already allocating a chunk */
>  	if (trans->allocating_chunk)
>  		return -ENOSPC;
> @@ -3736,9 +3736,17 @@ int btrfs_chunk_alloc(struct btrfs_trans_handle *trans, u64 flags,
>  	ret_bg = do_chunk_alloc(trans, flags);
>  	trans->allocating_chunk = false;
>  
> -	if (IS_ERR(ret_bg))
> +	if (IS_ERR(ret_bg)) {
>  		ret = PTR_ERR(ret_bg);
> -	else
> +	} else if (from_extent_allocation) {
> +		/*
> +		 * New block group is likely to be used soon. Try to activate
> +		 * it now. Failure is OK for now.
> +		 */
> +		btrfs_zone_activate(ret_bg);
> +	}
> +
> +	if (!ret)
>  		btrfs_put_block_group(ret_bg);
>  
>  	spin_lock(&space_info->lock);
> diff --git a/fs/btrfs/block-group.h b/fs/btrfs/block-group.h
> index 93aabc68bb6a..9c822367c432 100644
> --- a/fs/btrfs/block-group.h
> +++ b/fs/btrfs/block-group.h
> @@ -40,6 +40,7 @@ enum btrfs_chunk_alloc_enum {
>  	CHUNK_ALLOC_NO_FORCE,
>  	CHUNK_ALLOC_LIMITED,
>  	CHUNK_ALLOC_FORCE,
> +	CHUNK_ALLOC_FORCE_FOR_EXTENT,

There's a comment documenting what it means, I've written

	CHUNK_ALLOC_FORCE_FOR_EXTENT like CHUNK_ALLOC_FORCE but called from
	find_free_extent() that also activaes the zone

based on the changelog.