btrfs: use rbtree with leftmost node cached for tracking lowest block group
We keep track of the start offset of the block group with the lowest start offset at fs_info->first_logical_byte. This requires explicitly updating that field every time we add, delete or lookup a block group to/from the red black tree at fs_info->block_group_cache_tree. Since the block group with the lowest start address happens to always be the one that is the leftmost node of the tree, we can use a red black tree that caches the left most node. Then when we need the start address of that block group, we can just quickly get the leftmost node in the tree and extract the start offset of that node's block group. This avoids the need to explicitly keep track of that address in the dedicated member fs_info->first_logical_byte, and it also allows the next patch in the series to switch the lock that protects the red black tree from a spin lock to a read/write lock - without this change it would be tricky because block group searches also update fs_info->first_logical_byte. Reviewed-by: Nikolay Borisov <nborisov@suse.com> Signed-off-by: Filipe Manana <fdmanana@suse.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
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