Re: [PATCH v4 08/13] btrfs:zoned: make sb for npo2 zone devices align with sb log offsets

[Pankaj Raghav]

On 2022-05-17 14:42, David Sterba wrote:
> On Mon, May 16, 2022 at 06:54:11PM +0200, Pankaj Raghav wrote:
>> Superblocks for zoned devices are fixed as 2 zones at 0, 512GB and 4TB.
>> These are fixed at these locations so that recovery tools can reliably
>> retrieve the superblocks even if one of the mirror gets corrupted.
>>
>> power of 2 zone sizes align at these offsets irrespective of their
>> value but non power of 2 zone sizes will not align.
>>
>> To make sure the first zone at mirror 1 and mirror 2 align, write zero
>> operation is performed to move the write pointer of the first zone to
>> the expected offset. This operation is performed only after a zone reset
>> of the first zone, i.e., when the second zone that contains the sb is FULL.
> 
> Is it a good idea to do the "write zeros", instead of a plain "set write
> pointer"? I assume setting write pointer is instant, while writing
> potentially hundreds of megabytes may take significiant time. As the
> functions may be called from random contexts, the increased time may
> become a problem.
> 
Unfortunately it is not possible to just move the WP in zoned devices.
The only alternative that I could use is to do write zeroes which are
natively supported by some devices such as ZNS. It would be nice to know
if someone had a better solution to this instead of doing write zeroes
in zoned devices.

>> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
>> ---
>>  fs/btrfs/zoned.c | 68 ++++++++++++++++++++++++++++++++++++++++++++----
>>  1 file changed, 63 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
>> index 3023c871e..805aeaa76 100644
>> --- a/fs/btrfs/zoned.c
>> +++ b/fs/btrfs/zoned.c
>> @@ -760,11 +760,44 @@ int btrfs_check_mountopts_zoned(struct btrfs_fs_info *info)
>>  	return 0;
>>  }
>>  
>> +static int fill_sb_wp_offset(struct block_device *bdev, struct blk_zone *zone,
>> +			     int mirror, u64 *wp_ret)
>> +{
>> +	u64 offset = 0;
>> +	int ret = 0;
>> +
>> +	ASSERT(!is_power_of_two_u64(zone->len));
>> +	ASSERT(zone->wp == zone->start);
>> +	ASSERT(mirror != 0);
> 
> This could simply accept 0 as the mirror offset too, the calculation is
> trivial.
> 
Ok. I will fix it up!
>> +
>> +	switch (mirror) {
>> +	case 1:
>> +		div64_u64_rem(BTRFS_SB_LOG_FIRST_OFFSET >> SECTOR_SHIFT,
>> +			      zone->len, &offset);
>> +		break;
>> +	case 2:
>> +		div64_u64_rem(BTRFS_SB_LOG_SECOND_OFFSET >> SECTOR_SHIFT,
>> +			      zone->len, &offset);
>> +		break;
>> +	}
>> +
>> +	ret =  blkdev_issue_zeroout(bdev, zone->start, offset, GFP_NOFS, 0);
>> +	if (ret)
>> +		return ret;
>> +
>> +		/*
>> +		 * Non po2 zone sizes will not align naturally at
>> +		 * mirror 1 (512GB) and mirror 2 (4TB). The wp of the
>> +		 * 1st zone in those superblock mirrors need to be
>> +		 * moved to align at those offsets.
>> +		 */
> 
> Please move this comment to the helper fill_sb_wp_offset itself, there
> it's more discoverable.
> 
Ok.
>> +		is_sb_offset_write_req =
>> +			(zones_empty || (reset_zone_nr == 0)) && mirror &&
>> +			!is_power_of_2(zones[0].len);
> 
> Accepting 0 as the mirror number would also get rid of this wild
> expression substituting and 'if'.
> 
>>  
>>  		if (reset && reset->cond != BLK_ZONE_COND_EMPTY) {
>>  			ASSERT(sb_zone_is_full(reset));
>> @@ -795,6 +846,13 @@ static int sb_log_location(struct block_device *bdev, struct blk_zone *zones,
>>  			reset->cond = BLK_ZONE_COND_EMPTY;
>>  			reset->wp = reset->start;
>>  		}
>> +
>> +		if (is_sb_offset_write_req) {
> 
> And get rid of the conditional. The point of supporting both po2 and
> nonpo2 is to hide any implementation details to wrappers as much as
> possible.
> 
Alright. I will move the logic to the wrapper instead of having the
conditional in this function.
>> +			ret = fill_sb_wp_offset(bdev, &zones[0], mirror, &wp);
>> +			if (ret)
>> +				return ret;
>> +		}
>> +
>>  	} else if (ret != -ENOENT) {
>>  		/*
>>  		 * For READ, we want the previous one. Move write pointer to
Thanks for your comments.