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Message-ID: <9eb00b42-ca5b-c94e-319d-a0e102b99f02@opensource.wdc.com>
Date:   Mon, 9 May 2022 20:31:57 +0900
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Subject: Re: [PATCH v3 10/11] null_blk: allow non power of 2 zoned devices
Content-Language: en-US
To:     Pankaj Raghav <p.raghav@samsung.com>, jaegeuk@kernel.org,
        hare@suse.de, dsterba@suse.com, axboe@kernel.dk, hch@lst.de,
        snitzer@kernel.org
Cc:     Chris Mason <clm@fb.com>, Josef Bacik <josef@toxicpanda.com>,
        bvanassche@acm.org, linux-fsdevel@vger.kernel.org,
        matias.bjorling@wdc.com, Jens Axboe <axboe@fb.com>,
        gost.dev@samsung.com, jonathan.derrick@linux.dev,
        jiangbo.365@bytedance.com, linux-nvme@lists.infradead.org,
        dm-devel@redhat.com, Naohiro Aota <naohiro.aota@wdc.com>,
        linux-kernel@vger.kernel.org, Johannes Thumshirn <jth@kernel.org>,
        Sagi Grimberg <sagi@grimberg.me>,
        Alasdair Kergon <agk@redhat.com>, linux-block@vger.kernel.org,
        Chaitanya Kulkarni <kch@nvidia.com>,
        Keith Busch <kbusch@kernel.org>, linux-btrfs@vger.kernel.org,
        Luis Chamberlain <mcgrof@kernel.org>
References: <20220506081105.29134-1-p.raghav@samsung.com>
 <CGME20220506081116eucas1p2cce67bbf30f4c9c4e6854965be41b098@eucas1p2.samsung.com>
 <20220506081105.29134-11-p.raghav@samsung.com>
 <39a80347-af70-8af0-024a-52f92e27a14a@opensource.wdc.com>
 <aef68bcf-4924-8004-3320-325e05ca9b20@samsung.com>
From:   Damien Le Moal <damien.lemoal@opensource.wdc.com>
Organization: Western Digital Research
In-Reply-To: <aef68bcf-4924-8004-3320-325e05ca9b20@samsung.com>
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Precedence: bulk

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