btrfs: zoned: tweak reclaim threshold for zone capacity

With the introduction of zone capacity, the range [capacity, length] is
always zone unusable. Counting this region as a reclaim target will
cause reclaiming too early. Reclaim block groups based on bytes that can
be usable after resetting.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -2540,6 +2540,7 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 	u64 to_free, to_unusable;
 	const int bg_reclaim_threshold = READ_ONCE(fs_info->bg_reclaim_threshold);
 	bool initial = (size == block_group->length);
+	u64 reclaimable_unusable;
 
 	WARN_ON(!initial && offset + size > block_group->zone_capacity);
 
@@ -2570,12 +2571,15 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
 		spin_unlock(&block_group->lock);
 	}
 
+	reclaimable_unusable = block_group->zone_unusable -
+			       (block_group->length - block_group->zone_capacity);
 	/* All the region is now unusable. Mark it as unused and reclaim */
 	if (block_group->zone_unusable == block_group->length) {
 		btrfs_mark_bg_unused(block_group);
 	} else if (bg_reclaim_threshold &&
-		   block_group->zone_unusable >=
-		   div_factor_fine(block_group->length, bg_reclaim_threshold)) {
+		   reclaimable_unusable >=
+		   div_factor_fine(block_group->zone_capacity,
+				   bg_reclaim_threshold)) {
 		btrfs_mark_bg_to_reclaim(block_group);
 	}