Re: [PATCH v4 05/13] btrfs: zoned: Cache superblock location in btrfs_zoned_device_info

[David Sterba]

On Mon, May 16, 2022 at 06:54:08PM +0200, Pankaj Raghav wrote:
> Instead of calculating the superblock location every time, cache the
> superblock zone location in btrfs_zoned_device_info struct and use it to
> locate the zone index.
> 
> The functions such as btrfs_sb_log_location_bdev() and
> btrfs_reset_sb_log_zones() which work directly on block_device shall
> continue to use the sb_zone_number because btrfs_zoned_device_info
> struct might not have been initialized at that point.
> 
> This patch will enable non power-of-2 zoned devices to not perform
> division to lookup superblock and its mirror location.
> 
> Reviewed-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> ---
>  fs/btrfs/zoned.c | 13 +++++++++----
>  fs/btrfs/zoned.h |  1 +
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 06f22c021..e8c7cebb2 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -511,6 +511,11 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
>  			   max_active_zones - nactive);
>  	}
>  
> +	/* Cache the sb zone number */
> +	for (i = 0; i < BTRFS_SUPER_MIRROR_MAX; ++i) {
> +		zone_info->sb_zone_location[i] =
> +			sb_zone_number(zone_info->zone_size_shift, i);
> +	}

I don't think we need to cache the value right now, it's not in any hot
path and call to bdev_zone_no is relatively cheap (only dereferencing a
few pointers, all in-memory values).