btrfs: move out now unused BG from the reclaim list

[ Upstream commit a9f1897 ]

An unused block group is easy to remove to free up space and should be
reclaimed fast. Such block group can often already be a target of the
reclaim process. As we check list_empty(&bg->bg_list), we keep it in the
reclaim list. That block group is never reclaimed until the file system
is filled e.g. up to 75%.

Instead, we can move unused block group to the unused list and delete it
fast.

Fixes: 18bb8bb ("btrfs: zoned: automatically reclaim zones")
CC: stable@vger.kernel.org # 5.15+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/btrfs/block-group.c

@@ -1528,11 +1528,14 @@ void btrfs_mark_bg_unused(struct btrfs_block_group *bg)
 {
 	struct btrfs_fs_info *fs_info = bg->fs_info;
 
+	trace_btrfs_add_unused_block_group(bg);
 	spin_lock(&fs_info->unused_bgs_lock);
 	if (list_empty(&bg->bg_list)) {
 		btrfs_get_block_group(bg);
-		trace_btrfs_add_unused_block_group(bg);
 		list_add_tail(&bg->bg_list, &fs_info->unused_bgs);
+	} else {
+		/* Pull out the block group from the reclaim_bgs list. */
+		list_move_tail(&bg->bg_list, &fs_info->unused_bgs);
 	}
 	spin_unlock(&fs_info->unused_bgs_lock);
 }