Re: [PATCH v2] btrfs: zoned: don't wait for ordered extents in do_zone_finish()

["Naohiro Aota" <Naohiro.Aota@wdc.com>]

On Tue Oct 27, 2026 at 11:33 PM JST, Johannes Thumshirn wrote:
> In general, we want to ensure all writes in the block group to
> complete before finishing the zone, as relocation does. However, the
> current callers do not need the wait (or cannot wait) in do_zone_finish():
>
> - Relocation already waits for reservations and ordered extents itself
>   before calling btrfs_zone_finish().
> - btrfs_delete_unused_bgs() checks that ->reserved is zero and sets the
>   block group read-only while holding groups_sem, so there can be no
>   in-flight data write.
> - btrfs_zone_finish_one_bg() is called from the allocation path and
>   cannot wait.
>
> Dropping the wait is still safe. Once the block group is read-only,
> ->reserved can only drop, and for data it only drops after the ordered
> extent's I/O has completed. So if ->reserved is zero, no data write is
> left in flight. If it is non-zero, do_zone_finish() returns -EAGAIN,
> which all callers handle.
>
> This makes btrfs_zone_finish_one_bg() fail with -EAGAIN when it races
> with an allocation in the selected block group.

Oops, I meant to keep the pragraph mentioning the deadlock and the call
chain. I think we can get that part back on the commit.

Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>

>
> Fixes: 393f646e34c1 ("btrfs: zoned: finish least available block group on data bg allocation")
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
> Changes to v1:
> - Reword changelog to undermine why this is safe (Naohiro) 
>
>  fs/btrfs/zoned.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 1fa855a0c31a..35b422a53e2a 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -2604,9 +2604,10 @@ static int do_zone_finish(struct btrfs_block_group *block_group, bool fully_writ
>  
>  		/* Ensure all writes in this block group finish */
>  		btrfs_wait_block_group_reservations(block_group);
> -		/* No need to wait for NOCOW writers. Zoned mode does not allow that */
> -		btrfs_wait_ordered_roots(fs_info, U64_MAX, block_group);
> -		/* Wait for extent buffers to be written. */
> +		/*
> +		 * No need to wait for NOCOW writers, as zoned mode does not
> +		 * allow that, only wait for extent buffers to be written.
> +		 */
>  		if (is_metadata)
>  			wait_eb_writebacks(block_group);
>