Re: [PATCH v2 2/3] blkid: add magic and probing for zoned btrfs

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 14/04/2021 03:33, Naohiro Aota wrote:
> This commit adds zone-aware magics and probing functions for zoned btrfs.
> 
> Superblock (and its copies) is the only data structure in btrfs with a

The superblock?

> fixed location on a device. Since we cannot overwrite in a sequential write

cannot do overwrites

> required zone, we cannot place superblock in the zone.

the superblock

> 
> Thus, zoned btrfs use superblock log writing to update superblock on

Thus, zoned btrfs uses superblock log writing to update superblocks on

> sequential write required zones. It uses two zones as a circular buffer to
> write updated superblocks. Once the first zone is filled up, start writing
> into the second buffer. When both zones are filled up and before start

starting to write

> writing to the first zone again, it reset the first zone.
> 
> We can determine the position of the latest superblock by reading write

reading the write pointer

> pointer information from a device. One corner case is when both zones are
> full. For this situation, we read out the last superblock of each zone and
> compare them to determine which zone is older.
> 
> The magics can detect a superblock magic ("_BHRfs_M") at the beginning of
> zone #0 or zone #1 to see if it is zoned btrfs. When both zones are filled
> up, zoned btrfs reset the first zone to write a new superblock. If btrfs


resets

> crash at the moment, we do not see a superblock at zone #0. Thus, we need

crashes

> to check not only zone #0 but also zone #1.
> 
> It also supports temporary magic ("!BHRfS_M") in zone #0. The mkfs.btrfs

the temporary magic [...]. Mkfs.btrfs

[...]

> +	 * Log position:
> +	 *   *: Special case, no superblock is written
> +	 *   0: Use write pointer of zones[0]
> +	 *   1: Use write pointer of zones[1]
> +	 *   C: Compare super blcoks from zones[0] and zones[1], use the latest
                        blocks ~^

[...]

> +	rep_size = sizeof(struct blk_zone_report) + sizeof(struct blk_zone) * 2;
> +	rep = malloc(rep_size);
> +	if (!rep)
> +		return -errno;
> +
> +	memset(rep, 0, rep_size);

I think Damien already pointed this out, but that's a good opportunity for calloc().

Otherwise,
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Tested-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>