Re: [PATCH] btrfs: zoned: reset meta_write_pointer on zone reset

On Fri Jul 3, 2026 at 2:54 PM JST, Johannes Thumshirn wrote:
> btrfs_reset_unused_block_groups() resets a block group's zone and sets
> alloc_offset back to 0 so the space can be reused, but it leaves
> meta_write_pointer pointing at the previous end of the zone.
>
> Once the block group is reactivated and reused for metadata, newly
> allocated tree blocks live before that stale write pointer.
> btrfs_check_meta_write_pointer() then sees them behind the write pointer,
> so they can never be written out in sequential order: the dirty extent
> buffers are stranded and pin their btree_inode folios until unmount.
>
> Reset meta_write_pointer back to the start of the block group for
> metadata and system block groups.

Looks good to me,

Reviewed-by: Naohiro Aota <[email protected]>

>
> Fixes: 453a73c3069a ("btrfs: zoned: reclaim unused zone by zone resetting")
> Cc: [email protected]
> Signed-off-by: Johannes Thumshirn <[email protected]>
> ---
>  fs/btrfs/zoned.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index ab7c3cc52599..765655473263 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -3218,6 +3218,17 @@ int btrfs_reset_unused_block_groups(struct btrfs_space_info *space_info, u64 num
>  		reclaimed = bg->alloc_offset;
>  		bg->zone_unusable = bg->length - bg->zone_capacity;
>  		bg->alloc_offset = 0;
> +		/*
> +		 * The zone was just reset to empty, so alloc_offset went back to
> +		 * the start of the zone. For metadata/system block groups the
> +		 * write pointer must follow it back to the start of the zone;
> +		 * otherwise it stays stale at the previous (finished) zone end,
> +		 * and metadata written into the reused zone would sit behind the
> +		 * write pointer, could never be written out in sequential order,
> +		 * and would be stranded (pinning its folio) until unmount.
> +		 */
> +		if (bg->flags & (BTRFS_BLOCK_GROUP_METADATA | BTRFS_BLOCK_GROUP_SYSTEM))
> +			bg->meta_write_pointer = bg->start;
>  		/*
>  		 * This holds because we currently reset fully used then freed
>  		 * block group.