Re: [PATCH 6/7] btrfs: zoned: fix deadlock waiting for ticket during data relocation

[Boris Burkov <boris@bur.io>]

On Wed, May 13, 2026 at 02:34:44PM +0200, Johannes Thumshirn wrote:
> When performing data relocation on a zoned filesystem, BTRFS can deadlock
> in handle_reserve_tickets(). The relocation process is waiting on a space
> reservation ticket that can never be fulfilled, because the relocation
> itself is the operation responsible for freeing up that space.
> 
> Fix this by introducing a new flush state,
> BTRFS_RESERVE_FLUSH_DATA_RELOCATION, specifically for data chunk
> allocation during zoned relocation. Like
> BTRFS_RESERVE_FLUSH_FREE_SPACE_INODE, this state uses
> priority_reclaim_data_space() instead of the normal flushing path, which
> avoids re-entering the relocation code and breaking the deadlock cycle.
> 
> In btrfs_alloc_data_chunk_ondemand(), select this new flush state when the
> inode belongs to a data relocation root on a zoned filesystem.

This looks good, but a general question: are any of the other data
flushers valid to run at this point or only chunk allocation?

> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/btrfs/delalloc-space.c |  2 ++
>  fs/btrfs/space-info.c     |  2 ++
>  fs/btrfs/space-info.h     | 11 +++++++++++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/fs/btrfs/delalloc-space.c b/fs/btrfs/delalloc-space.c
> index 0970799d0aa4..c9d3ec6bbc3c 100644
> --- a/fs/btrfs/delalloc-space.c
> +++ b/fs/btrfs/delalloc-space.c
> @@ -134,6 +134,8 @@ int btrfs_alloc_data_chunk_ondemand(const struct btrfs_inode *inode, u64 bytes)
>  
>  	if (btrfs_is_free_space_inode(inode))
>  		flush = BTRFS_RESERVE_FLUSH_FREE_SPACE_INODE;
> +	else if (btrfs_is_zoned(fs_info) && btrfs_is_data_reloc_root(root))
> +		flush = BTRFS_RESERVE_FLUSH_DATA_RELOCATION;
>  
>  	return btrfs_reserve_data_bytes(data_sinfo_for_inode(inode), bytes, flush);
>  }
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index 58256a9c056d..ec811a77ebb1 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -1703,6 +1703,7 @@ static int handle_reserve_ticket(struct btrfs_space_info *space_info,
>  						ARRAY_SIZE(evict_flush_states));
>  		break;
>  	case BTRFS_RESERVE_FLUSH_FREE_SPACE_INODE:
> +	case BTRFS_RESERVE_FLUSH_DATA_RELOCATION:
>  		priority_reclaim_data_space(space_info, ticket);
>  		break;
>  	default:
> @@ -1966,6 +1967,7 @@ int btrfs_reserve_data_bytes(struct btrfs_space_info *space_info, u64 bytes,
>  
>  	ASSERT(flush == BTRFS_RESERVE_FLUSH_DATA ||
>  	       flush == BTRFS_RESERVE_FLUSH_FREE_SPACE_INODE ||
> +	       flush == BTRFS_RESERVE_FLUSH_DATA_RELOCATION ||
>  	       flush == BTRFS_RESERVE_NO_FLUSH, "flush=%d", flush);
>  	ASSERT(!current->journal_info || flush != BTRFS_RESERVE_FLUSH_DATA,
>  	       "current->journal_info=0x%lx flush=%d",
> diff --git a/fs/btrfs/space-info.h b/fs/btrfs/space-info.h
> index 24f45072ca4b..f2b8be2af5c3 100644
> --- a/fs/btrfs/space-info.h
> +++ b/fs/btrfs/space-info.h
> @@ -77,6 +77,17 @@ enum btrfs_reserve_flush_enum {
>  	 */
>  	BTRFS_RESERVE_FLUSH_ALL_STEAL,
>  
> +	/*
> +	 * This is for relocation on zoned filesystems only. We need to use
> +	 * priority flushing for this, because otherwise we can deadlock on
> +	 * waiting for a ticket, that cannot be granted, because we cannot do
> +	 * any allocations.
> +	 *
> +	 * Apart from being specific to zoned relocation, it is equal to
> +	 * BTRFS_FLUSH_FREE_SPACE_INODE.
> +	 */

Should it be named ZONED_RELOCATION then?

> +	BTRFS_RESERVE_FLUSH_DATA_RELOCATION,
> +
>  	/*
>  	 * This is for btrfs_use_block_rsv only.  We have exhausted our block
>  	 * rsv and our global block rsv.  This can happen for things like
> -- 
> 2.54.0
>