Re: [PATCH 4/4] block: null_blk: Improve device creation with configfs

[Jens Axboe <axboe@kernel.dk>]

On 4/19/22 5:00 AM, Damien Le Moal wrote:
> Currently, the directory name used to create a nullb device through
> sysfs is not used as the device name, potentially causing headaches for
> users if devices are already created through the modprobe operation
> withe the nr_device module parameter not set to 0. E.g. a user can do
> "mkdir /sys/kernel/config/nullb/nullb0" to create a nullb device while
> /dev/nullb0 wasalready created from modprobe. In this case, the configfs
                ^^^

space

> nullb device will be named nullb1, causing confusion for the user.
> 
> Simplify this by using the configfs directory name as the nullb device
> name, always, unless another nullb device is already using the same
> name. E.g. if modprobe created nullb0, then:
> 
> $ mkdir /sys/kernel/config/nullb/nullb0
> mkdir: cannot create directory '/sys/kernel/config/nullb/nullb0': File
> exists
> 
> will be reported to th user.
> 
> To implement this, the function null_find_dev_by_name() is added to
> check for the existence of a nullb device with the name used for a new
> configfs device directory. nullb_group_make_item() uses this new
> function to check if the directory name can be used as the disk name.
> Finally, null_add_dev() is modified to use the device config item name
> as the disk name for new nullb device, for devices created using
> configfs. The naming of devices created though modprobe remains
> unchanged.
> 
> Of note is that it is possible for a user to create through configfs a
> nullb device with the same name as an existing device. E.g.

This is nice, and solves both the confusing part of having
pre-configured devices, but also using the actual directory name as the
device name even if they are not ordered.

Only odd bit is you can create a device name where a special file of
that name already exists, but I don't think that's solvable in a clean
way and we just need to ignore that. That's arguably a user error, don't
pick names that already exist.

-- 
Jens Axboe