What devices do /sys/class/bdi/ settings correspond to

/sys/class/bdi has settings for things like readahead and dirty page ratios for devices. It is organized by bdi identifier, which is the major:minor device number or the st_dev value set by the file system. My /sys/class/bdi contains:

My question is: how do I know what devices/file systems the 0: entries correspond to? The entries with non-zero major numbers are devices listed in e.g. /proc/devices, but I can't figure out what the other entries are for.