Re: [PATCH v3 11/11] dm-zoned: ensure only power of 2 zone sizes are allowed

[Pankaj Raghav <p.raghav@samsung.com>]

Hi David,

On 2022-05-09 20:54, David Sterba wrote:>> diff --git
a/drivers/md/dm-zone.c b/drivers/md/dm-zone.c
>> index 3e7b1fe15..27dc4ddf2 100644
>> --- a/drivers/md/dm-zone.c
>> +++ b/drivers/md/dm-zone.c
>> @@ -231,6 +231,18 @@ static int dm_revalidate_zones(struct mapped_device *md, struct dm_table *t)
>>  	struct request_queue *q = md->queue;
>>  	unsigned int noio_flag;
>>  	int ret;
>> +	struct block_device *bdev = md->disk->part0;
>> +	sector_t zone_sectors;
>> +	char bname[BDEVNAME_SIZE];
>> +
>> +	zone_sectors = bdev_zone_sectors(bdev);
>> +
>> +	if (!is_power_of_2(zone_sectors)) {
> 
> is_power_of_2 takes 'unsigned long' and sector_t is u64, so this is not
> 32bit clean and we had an actual bug where value 1<<48 was not
> recognized as power of 2.
> 
Good catch. Now I understand why btrfs has a helper for is_power_of_two_u64.

But the zone size can never be more than 32bit value so the zone size
sect will never greater than unsigned long.

With that said, we have two options:

1.) We can put a comment explaining that even though it is 32 bit
unsafe, zone size sect can never be a 32bit value

or

2) We should move the btrfs only helper `is_power_of_two_u64` to some
common header and use it everywhere.

Let me know your thoughts.