Nullblk configfs oddities

[Josef Bacik <josef@toxicpanda.com>]

Hello,

I'm trying to add a test to fsperf and it requires the use of nullblk.  I'm
trying to use the configfs thing, and it's doing some odd things.  My basic
reproducer is

modprobe null_blk
mkdir /sys/kernel/config/nullb/nullb0
echo some shit into the config
echo 1 > /sys/kernel/config/nullb/nullb0/power

Now null_blk apparently defaults to nr_devices == 1, so it creates nullb0 on
modprobe.  But this doesn't show up in the configfs directory.  There's no way
to find this out until when I try to mkfs my nullb0 and it doesn't work.  The
above steps gets my device created at /dev/nullb1, but there's no actual way to
figure out that's what happened.  If I do something like
/sys/kernel/config/nullb/nullbfsperf I still just get nullb<number>, I don't get
my fancy name.

I don't really care what the solution looks like, I either need to be able to
create a random name and have a symlink automatically created so I can get to
the right device node (which I don't think can be done without udev, so I'm not
fan of this) or I need a way to figure out from configfs which device my thing
actually got created as.

It would also be nice if /sys/kernel/config/nullb/nullb0 was populated at
modinfo time so I know it's there already, then I could do something silly like
get the next number and use that as my device.  Thanks,

Josef