Re: Nullblk configfs oddities

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

On 4/18/22 14:38, Josef Bacik wrote:
> 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.
> 

when you load module with default module parameter it will create a 
default device with no memory backed mode, that will not be visible in 
the configfs.

So you need to load the module with nr_devices=0 that will prevent the 
null_blk to create the default device which is not memory backed and not 
present in the configfs:-

linux-block (for-next) # modprobe null_blk
linux-block (for-next) # lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda       8:0    0   50G  0 disk
├─sda1    8:1    0    1G  0 part /boot
└─sda2    8:2    0   49G  0 part /home
sdb       8:16   0  100G  0 disk /mnt/data
sr0      11:0    1 1024M  0 rom
nullb0  250:0    0  250G  0 disk <-------------------
zram0   251:0    0    8G  0 disk [SWAP]
vda     252:0    0  512M  0 disk
nvme0n1 259:0    0    1G  0 disk
linux-block (for-next) # tree config
config
└── nullb
     └── features

1 directory, 1 file
linux-block (for-next) # modprobe  -r null_blk
linux-block (for-next) # modprobe null_blk nr_devices=0
linux-block (for-next) # lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda       8:0    0   50G  0 disk
├─sda1    8:1    0    1G  0 part /boot
└─sda2    8:2    0   49G  0 part /home
sdb       8:16   0  100G  0 disk /mnt/data
sr0      11:0    1 1024M  0 rom
zram0   251:0    0    8G  0 disk [SWAP]
vda     252:0    0  512M  0 disk
nvme0n1 259:0    0    1G  0 disk
linux-block (for-next) # tree config
config
└── nullb
     └── features

1 directory, 1 file
linux-block (for-next) #

-ck