On 4/18/22 4:21 PM, Chaitanya Kulkarni wrote: > On 4/18/22 15:14, Jens Axboe wrote: >> On 4/18/22 3:54 PM, Chaitanya Kulkarni wrote: >>> 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. >> >> Right, the problem is really that pre-configured devices (via nr_devices >> being bigger than 0, which is the default) don't show up in configfs. >> That, to me, is the real issue here, because it means you need to know >> which ones are already setup before doing mkdir for a new one. >> >> On top of that, it's also odd that they don't show up there to begin >> with. >> > > it is indeed confusing, maybe we need to find a way to populate the > configfs when loading the module? but I'm not sure if that is > the right approach since configs ideally should be populated by > user. > > OTOH we can make the memory_backed module param [1] so user can > tentatively not use configfs and only rely on default configuration ? Arguably configfs should just be disabled if loading with nr_devices larger than 0, as it's a mess of an API as it stands. But probably too late for that. The fact that we also have an option that's specific to configfs just makes it even worse. I don't know much about configfs, but pre-populating with the configured devices and options would be ideal and completely solve this. I think that would be the best solution given the current situation. Not that it's THAT important, null_blk is a developer tool and as such can have some sharper and rouger edges. Still would be nice to make it saner, though. -- Jens Axboe
