btrfs: reset block group size class when it becomes empty

Block group size classes are managed consistently everywhere.
Currently, btrfs_use_block_group_size_class() sets a block group's size
class to specialize it for a specific allocation size. However, this
size class remains "stale" even if the block group becomes completely
empty (both used and reserved bytes reach zero).

This happens in two scenarios:

1. When space reservations are freed (e.g., due to errors or transaction
   aborts) via btrfs_free_reserved_bytes().
2. When the last extent in a block group is freed via
   btrfs_update_block_group().

While size classes are advisory, a stale size class can cause
find_free_extent to unnecessarily skip candidate block groups during
initial search loops. This undermines the purpose of size classes to
reduce fragmentation by keeping block groups restricted to a specific
size class when they could be reused for any size.

Fix this by resetting the size class to BTRFS_BG_SZ_NONE whenever a
block group's used and reserved counts both reach zero. This ensures
that empty block groups are fully available for any allocation size in
the next cycle.

Fixes: 52bb7a2 ("btrfs: introduce size class to block group allocator")
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: Jiasheng Jiang

fs/btrfs/block-group.c

@@ -3760,6 +3760,14 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans)
 	return ret;
 }
 
+static void btrfs_maybe_reset_size_class(struct btrfs_block_group *bg)
+{
+	lockdep_assert_held(&bg->lock);
+	if (btrfs_block_group_should_use_size_class(bg) &&
+	    bg->used == 0 && bg->reserved == 0)
+		bg->size_class = BTRFS_BG_SZ_NONE;
+}
+
 int btrfs_update_block_group(struct btrfs_trans_handle *trans,
 			     u64 bytenr, u64 num_bytes, bool alloc)
 {
@@ -3824,6 +3832,7 @@ int btrfs_update_block_group(struct btrfs_trans_handle *trans,
 		old_val -= num_bytes;
 		cache->used = old_val;
 		cache->pinned += num_bytes;
+		btrfs_maybe_reset_size_class(cache);
 		btrfs_space_info_update_bytes_pinned(space_info, num_bytes);
 		space_info->bytes_used -= num_bytes;
 		space_info->disk_used -= num_bytes * factor;
@@ -3952,6 +3961,7 @@ void btrfs_free_reserved_bytes(struct btrfs_block_group *cache, u64 num_bytes,
 	spin_lock(&cache->lock);
 	bg_ro = cache->ro;
 	cache->reserved -= num_bytes;
+	btrfs_maybe_reset_size_class(cache);
 	if (is_delalloc)
 		cache->delalloc_bytes -= num_bytes;
 	spin_unlock(&cache->lock);