Re: [PATCH v3 10/11] null_blk: allow non power of 2 zoned devices

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 2022/05/09 20:06, Pankaj Raghav wrote:
> 
>>> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
>>> index 5cb4c92cd..ed9a58201 100644
>>> --- a/drivers/block/null_blk/main.c
>>> +++ b/drivers/block/null_blk/main.c
>>> @@ -1929,9 +1929,8 @@ static int null_validate_conf(struct nullb_device *dev)
>>>  	if (dev->queue_mode == NULL_Q_BIO)
>>>  		dev->mbps = 0;
>>>  
>>> -	if (dev->zoned &&
>>> -	    (!dev->zone_size || !is_power_of_2(dev->zone_size))) {
>>> -		pr_err("zone_size must be power-of-two\n");
>>> +	if (dev->zoned && !dev->zone_size) {
>>> +		pr_err("zone_size must not be zero\n");
>>
>> May be a simpler phrasing would be better:
>>
>> pr_err("Invalid zero zone size\n");
>>
> Ack. I will change this in the next rev.
>>>  		return -EINVAL;
>>>  	}
>>>  
>>> diff --git a/drivers/block/null_blk/zoned.c b/drivers/block/null_blk/zoned.c
>>> index dae54dd1a..00c34e65e 100644
>>> --- a/drivers/block/null_blk/zoned.c
>>> +++ b/drivers/block/null_blk/zoned.c
>>> @@ -13,7 +13,10 @@ static inline sector_t mb_to_sects(unsigned long mb)
>>>  
>>>  static inline unsigned int null_zone_no(struct nullb_device *dev, sector_t sect)
>>>  {
>>> -	return sect >> ilog2(dev->zone_size_sects);
>>> +	if (is_power_of_2(dev->zone_size_sects))
>>> +		return sect >> ilog2(dev->zone_size_sects);
>>
>> As a separate patch, I think we should really have ilog2(dev->zone_size_sects)
>> as a dev field to avoid doing this ilog2 for every call..
>>
> I don't think that is possible because `zone_size_sects` can also be non
> po2.

But when it is we can optimize that. All we need is add a "zone_size_sect_shift"
field that is initialized when zone_size_sects is set when the device is
created. Then, you can have code like:

	if (dev->zone_size_sect_shift))
		return sect >> dev->zone_size_sect_shift;

Which avoids both is_power_of_2() and ilog2() calls for every IO.

-- 
Damien Le Moal
Western Digital Research