Re: Nullblk configfs oddities

[Jens Axboe <axboe@kernel.dk>]

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