Re: [PATCH] btrfs: zoned: fixup last alloc pointer after extent removal for RAID1

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

On Wed Dec 17, 2025 at 7:34 PM JST, Naohiro Aota wrote:
> When a block group is composed of a sequential write zone and a conventional
> zone, we recover the (pseudo) write pointer of the conventional zone using the
> end of the last allocated position.
>
> However, if the last extent in a block group is removed, the last extent
> position will be smaller than the other real write pointer position. Then, that
> will cause an error due to mismatch of the write pointers.
>
> We can fixup this case by moving the alloc_offset to the corresponding write
> pointer position.
>
> Fixes: 568220fa9657 ("btrfs: zoned: support RAID0/1/10 on top of raid stripe tree")
> CC: stable@vger.kernel.org # 6.12+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>  fs/btrfs/zoned.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 359a98e6de85..17dde95eb3e7 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -1490,6 +1490,23 @@ static int btrfs_load_block_group_raid1(struct btrfs_block_group *bg,
>  	/* In case a device is missing we have a cap of 0, so don't use it. */
>  	bg->zone_capacity = min_not_zero(zone_info[0].capacity, zone_info[1].capacity);
>  
> +	if (last_alloc) {

Oops, we don't need this "if" because we need to run this even if we
have no extents (alloc_offset == 0) and the write pointer > 0.

> +		/*
> +		 * When the last extent is removed, last_alloc can be smaller
> +		 * than the other write pointer. In that case, last_alloc should
> +		 * be moved to the corresponding write pointer position.
> +		 */
> +		for (i = 0; i < map->num_stripes; i++) {
> +			if (zone_info[i].alloc_offset == WP_MISSING_DEV ||
> +			    zone_info[i].alloc_offset == WP_CONVENTIONAL)
> +				continue;
> +			if (last_alloc <= zone_info[i].alloc_offset) {
> +				last_alloc = zone_info[i].alloc_offset;
> +				break;
> +			}
> +		}
> +	}
> +
>  	for (i = 0; i < map->num_stripes; i++) {
>  		if (zone_info[i].alloc_offset == WP_MISSING_DEV)
>  			continue;