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
