Re: [PATCH] btrfs: zoned: fix zone_unusable accounting on making BG RW again

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Jul 11, 2024 at 12:23:54AM +0900, Naohiro Aota wrote:
> When btrfs makes a block group read-only, it adds all free regions in the
> BG to space_info->bytes_readonly. That free space excludes reserved and
> pinned regions. OTOH, when btrfs makes the BG read-write again, it moves
> all the unused regions into the block group's zone_unusable. That unused
> region includes reserved and pinned regions. As a result, it counts too
> much zone_unusable bytes.
> 
> Fortunately (or unfortunately), having erroneous zone_unusable does not
> affect the calculation of space_info->bytes_readonly, because free
> space (num_bytes in btrfs_dec_block_group_ro) calculation is done based on
> the erroneous zone_unusable and it reduces the num_bytes just to cancel the
> error.
> 
> This behavior can be easily discovered by adding a WARN_ON to check e.g,
> "bg->pinned > 0" in btrfs_dec_block_group_ro(), and running fstests test
> case like btrfs/282.
> 
> Fix it by properly considering pinned and reserved in
> btrfs_dec_block_group_ro(). Also, add a WARN_ON and introduce
> btrfs_space_info_update_bytes_zone_unusable() to catch a similar mistake.
> 
> Fixes: 169e0da91a21 ("btrfs: zoned: track unusable bytes for zones")
> CC: stable@vger.kernel.org # 5.15+
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef