btrfs: block-group: fix free-space bitmap threshold

[BUG]
After commit 9afc6649 ("btrfs: block-group: refactor how we read one
block group item"), cache->length is being assigned after calling
btrfs_create_block_group_cache. This causes a problem since
set_free_space_tree_thresholds calculates the free-space threshold to
decide if the free-space tree should convert from extents to bitmaps.

The current code calls set_free_space_tree_thresholds with cache->length
being 0, which then makes cache->bitmap_high_thresh zero. This implies
the system will always use bitmap instead of extents, which is not
desired if the block group is not fragmented.

This behavior can be seen by a test that expects to repair systems
with FREE_SPACE_EXTENT and FREE_SPACE_BITMAP, but the current code only
created FREE_SPACE_BITMAP.

[FIX]
Call set_free_space_tree_thresholds after setting cache->length. There
is now a WARN_ON in set_free_space_tree_thresholds to help preventing
the same mistake to happen again in the future.

Link: https://github.com/kdave/btrfs-progs/issues/251
Fixes: 9afc6649 ("btrfs: block-group: refactor how we read one block group item")
CC: stable@vger.kernel.org # 5.8+
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -1798,7 +1798,6 @@ static struct btrfs_block_group *btrfs_create_block_group_cache(
 
 	cache->fs_info = fs_info;
 	cache->full_stripe_len = btrfs_full_stripe_len(fs_info, start);
-	set_free_space_tree_thresholds(cache);
 
 	cache->discard_index = BTRFS_DISCARD_INDEX_UNUSED;
 
@@ -1912,6 +1911,8 @@ static int read_one_block_group(struct btrfs_fs_info *info,
 	if (ret < 0)
 		goto error;
 
+	set_free_space_tree_thresholds(cache);
+
 	if (need_clear) {
 		/*
 		 * When we mount with old space cache, we need to
@@ -2132,6 +2133,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans, u64 bytes_used,
 		return -ENOMEM;
 
 	cache->length = size;
+	set_free_space_tree_thresholds(cache);
 	cache->used = bytes_used;
 	cache->flags = type;
 	cache->last_byte_to_unpin = (u64)-1;

fs/btrfs/free-space-tree.c

@@ -22,6 +22,10 @@ void set_free_space_tree_thresholds(struct btrfs_block_group *cache)
 	size_t bitmap_size;
 	u64 num_bitmaps, total_bitmap_size;
 
+	if (WARN_ON(cache->length == 0))
+		btrfs_warn(cache->fs_info, "block group %llu length is zero",
+			   cache->start);
+
 	/*
 	 * We convert to bitmaps when the disk space required for using extents
 	 * exceeds that required for using bitmaps.