Re: [PATCH 1/2] btrfs: zoned: use rcu list for iterating devices to collect stats

[David Sterba <dsterba@suse.cz>]

On Mon, Jan 22, 2024 at 02:51:03AM -0800, Johannes Thumshirn wrote:
> As btrfs_zoned_should_reclaim only has to iterate the device list in order
> to collect stats on the device's total and used bytes, we don't need to
> take the full blown mutex, but can iterate the device list in a rcu_read
> context.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/btrfs/zoned.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 168af9d000d1..b7e7b5a5a6fa 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -2423,15 +2423,15 @@ bool btrfs_zoned_should_reclaim(struct btrfs_fs_info *fs_info)
>  	if (fs_info->bg_reclaim_threshold == 0)
>  		return false;
>  
> -	mutex_lock(&fs_devices->device_list_mutex);
> -	list_for_each_entry(device, &fs_devices->devices, dev_list) {
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
>  		if (!device->bdev)
>  			continue;
>  
>  		total += device->disk_total_bytes;
>  		used += device->bytes_used;
>  	}
> -	mutex_unlock(&fs_devices->device_list_mutex);
> +	rcu_read_unlock();

This is basically only a hint and inaccuracies in the total or used
values would be transient, right? The sum is calculated each time the
funciton is called, not stored anywhere so in the unlikely case of
device removal it may skip reclaim once, but then pick it up later.
Any actual removal of the block groups in verified again and properly
locked in btrfs_reclaim_bgs_work().