Re: [PATCH 3/3] btrfs: zoned: limit active zones to max_open_zones

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

On Mon Jul 7, 2025 at 12:40 PM JST, Damien Le Moal wrote:
> On 7/7/25 11:44 AM, Naohiro Aota wrote:
>> When there is no active zone limit, we can technically write into any
>> number of zones at the same time. However, exceeding the max open zones can
>> degrade performance. To prevent this, set the max_active_zones to
>> bdev_max_open_zones() if there is no active zone limit.
>> 
>> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>> ---
>>  fs/btrfs/zoned.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
>> index 9c354e84ab07..bdcfabcb35e7 100644
>> --- a/fs/btrfs/zoned.c
>> +++ b/fs/btrfs/zoned.c
>> @@ -418,6 +418,8 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device, bool populate_cache)
>>  		zone_info->nr_zones++;
>>  
>>  	max_active_zones = bdev_max_active_zones(bdev);
>> +	if (!max_active_zones)
>> +		max_active_zones = bdev_max_open_zones(bdev);
>
> 	max_active_zones = min_not_zero(bdev_max_active_zones(bdev),
> 					bdev_max_open_zones(bdev));
>
> And what if the device has no limits at all ? (e.g. no max active limit and no
> max open limit). In this case, max_active_zones will be zero. Should we perhaps
> set in that case a default max_active zones ?
>
> For information, the block layer zone write plugging defaults to create a 128
> zones pull of write plugs if the device has no limits
> (BLK_ZONE_WPLUG_DEFAULT_POOL_SIZE in block/blk-zoned.c). I would recommend
> using that limit here too. So something like this:
>
> /* Default number of max active zones when the device has no limits. */
> #define BTRFS_ZONE_DEFAULT_MAX_ACTIVE	128
>
> 	max_active_zones = min_not_zero(bdev_max_active_zones(bdev),
> 					bdev_max_open_zones(bdev));
> 	if (!max_active_zones &&
> 	    bdev_nr_zones(bdev) > BTRFS_ZONE_DEFAULT_MAX_ACTIVE)
> 		max_active_zones = BTRFS_ZONE_DEFAULT_MAX_ACTIVE;

Indeed. I'll do so in the next version.

>
>
>>  	if (max_active_zones && max_active_zones < BTRFS_MIN_ACTIVE_ZONES) {
>>  		btrfs_err(fs_info,
>>  "zoned: %s: max active zones %u is too small, need at least %u active zones",