Re: [PATCH v2 3/4] btrfs: zoned: drop stranded dirty metadata buffers at unmount

On Thu, Jul 23, 2026 at 05:54:27PM +0200, Johannes Thumshirn wrote:
> On a zoned filesystem a freed tree block is kept dirty and flagged
> EXTENT_BUFFER_ZONED_ZEROOUT so a later writeback zeroes it out and
> advances the zone write pointer. Unsynced tree-log updates (e.g. from
> rename or link) leave such buffers behind when the log is freed at
> commit, and across log generations they can end up ahead of the write
> pointer behind a hole, so btree_writepages() can never write them.
> During normal operation the space is later reclaimed by a zone reset; at
> unmount it is not, and the buffers survive to the final iput() of the
> btree inode, which hangs in folio_wait_writeback() once the endio
> workqueues are stopped.
> 
> They cannot be written back from where they are freed (free_log_tree(),
> inside the committing transaction) without deadlocking against that
> commit, and they are stale anyway, not referenced by the committed
> superblock. Drop their dirty state in close_ctree(), before
> btrfs_stop_all_workers().
> 

Immediate fix looks fine, but the underlying issue is a little hacky.
Can we detect that we have staled them from free_log_tree() and release
the dirty there? Am I correct in understanding that this means we can
overestimate dirty? By an arbitrary amount if I do lots of unsynced
tree log updates?

Either way,
Reviewed-by: Boris Burkov

> Signed-off-by: Johannes Thumshirn <[email protected]>
> ---
>  fs/btrfs/disk-io.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 8d0afc2842cc..01d01e10d000 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -4549,6 +4549,13 @@ void __cold close_ctree(struct btrfs_fs_info *fs_info)
>  	free_root_pointers(fs_info, true);
>  	btrfs_free_fs_roots(fs_info);
>  
> +	/*
> +	 * Drop metadata left stranded ahead of a zone write pointer while the
> +	 * endio workqueues are still up, so the final iput() of the btree inode
> +	 * below does not hang submitting a write that can no longer complete.
> +	 */
> +	btrfs_zoned_release_dirty_metadata(fs_info);
> +
>  	/*
>  	 * We must make sure there is not any read request to
>  	 * submit after we stop all workers.
> -- 
> 2.55.0
>