Re: [PATCH v4 02/13] block: allow blk-zoned devices to have non-power-of-2 zone size

From: Pankaj Raghav <p.raghav@samsung.com>
To: <damien.lemoal@opensource.wdc.com>
Cc: <linux-nvme@lists.infradead.org>, <linux-fsdevel@vger.kernel.org>,
	<linux-btrfs@vger.kernel.org>, <jiangbo.365@bytedance.com>,
	<linux-block@vger.kernel.org>, <gost.dev@samsung.com>,
	<linux-kernel@vger.kernel.org>, <dm-devel@redhat.com>,
	Luis Chamberlain <mcgrof@kernel.org>, <axboe@kernel.dk>,
	<pankydev8@gmail.com>, <dsterba@suse.com>, <hch@lst.de>
Subject: Re: [PATCH v4 02/13] block: allow blk-zoned devices to have non-power-of-2 zone size
Date: Mon, 16 May 2022 21:05:32 +0200	[thread overview]
Message-ID: <b9a02707-8628-b7de-487b-a93697e8a2cb@samsung.com> (raw)
In-Reply-To: <20220516165416.171196-3-p.raghav@samsung.com>

Hi Damien,
I copied your comments from the previous thread to avoid confusion.

On 2022-05-16 16:00, Damien Le Moal wrote:
> On 2022/05/16 15:39, Pankaj Raghav wrote:
>> Checking if a given sector is aligned to a zone is a common
>> operation that is performed for zoned devices. Add
>> blk_queue_is_zone_start helper to check for this instead of opencoding it
>> everywhere.
>>
>> Convert the calculations on zone size to be generic instead of relying on
>> power_of_2 based logic in the block layer using the helpers wherever
>> possible.
>>
>> @@ -490,14 +490,29 @@ static int blk_revalidate_zone_cb(struct blk_zone *zone, unsigned int idx,
>>  	 * smaller last zone.
>>  	 */
>>  	if (zone->start == 0) {
>> -		if (zone->len == 0 || !is_power_of_2(zone->len)) {
>> -			pr_warn("%s: Invalid zoned device with non power of two zone size (%llu)\n",
>> -				disk->disk_name, zone->len);
>> +		if (zone->len == 0) {
>> +			pr_warn("%s: Invalid zone size",
>> +				disk->disk_name);
>
> This fits on one line, no ?
>
Yeah. I don't know why my formatter decided to do that. I will fix it. Thanks.
>> +			return -ENODEV;
>> +		}
>> +
>> +		/*
>> +		 * Don't allow zoned device with non power_of_2 zone size with
>> +		 * zone capacity less than zone size.
>> +		 */
>> +		if (!is_power_of_2(zone->len) &&
>> +		    zone->capacity < zone->len) {
>> +			pr_warn("%s: Invalid zoned size with non power of 2 zone size and zone capacity < zone size",
>> +				disk->disk_name);
>
> Very long... What about:
>
> pr_warn("%s: Invalid zone capacity for non power of 2 zone size",
> 	disk->disk_name);
>
This looks better. I will fix it up!
>>  			return -ENODEV;
>>  		}
>>
>>  		args->zone_sectors = zone->len;
>> -		args->nr_zones = (capacity + zone->len - 1) >> ilog2(zone->len);
>> +		/*
>> +		 * Division is used to calculate nr_zones for both power_of_2
>> +		 * and non power_of_2 zone sizes as it is not in the hot path.
>> +		 */
>
> This comment is not very useful.
>

I also saw you mentioning the comment was not useful in nvme code for
a similar scenario. Hannes brought up a point about making it
explicit when we are not using any special path for power of 2 zone sizes as in
most cases we do branching if the zone size is power of 2 to avoid division.

>> +		args->nr_zones = div64_u64(capacity + zone->len - 1, zone->len);
>>  	} else if (zone->start + args->zone_sectors < capacity) {
>>  		if (zone->len != args->zone_sectors) {
>>  			pr_warn("%s: Invalid zoned device with non constant zone size\n",
>> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
>> index 22fe512ee..32d7bd7b1 100644
>> --- a/include/linux/blkdev.h
>> +++ b/include/linux/blkdev.h
>> @@ -686,6 +686,22 @@ static inline unsigned int blk_queue_zone_no(struct request_queue *q,
>>  	return div64_u64(sector, zone_sectors);
>>  }
>>
>> +static inline bool blk_queue_is_zone_start(struct request_queue *q, sector_t sec)
>> +{
>> +	sector_t zone_sectors = blk_queue_zone_sectors(q);
>> +	u64 remainder = 0;
>> +
>> +	if (!blk_queue_is_zoned(q))
>> +		return false;
>> +
>> +	if (is_power_of_2(zone_sectors))
>> +		return IS_ALIGNED(sec, zone_sectors);
>> +
>> +	div64_u64_rem(sec, zone_sectors, &remainder);
>> +	/* if there is a remainder, then the sector is not aligned */
>
> Hmmm... Fairly obvious. Not sure this comment is useful.
>
True. I will remove it.

>> +	return remainder == 0;
>> +}
>> +
>>  static inline bool blk_queue_zone_is_seq(struct request_queue *q,
>>  					 sector_t sector)
>>  {
>> @@ -732,6 +748,12 @@ static inline unsigned int blk_queue_zone_no(struct request_queue *q,
>>  {
>>  	return 0;
>>  }
>> +
>> +static inline bool blk_queue_is_zone_start(struct request_queue *q, sector_t sec)
>> +{
>> +	return false;
>> +}
>> +
>>  static inline unsigned int queue_max_open_zones(const struct request_queue *q)
>>  {
>>  	return 0;
>
>