Re: simplify gendisk lookup and remove struct block_device aliases v4

[Christoph Hellwig]

Jens, can you take a look and possibly pick this series up?

On Thu, Oct 29, 2020 at 03:58:23PM +0100, Christoph Hellwig wrote:
> Hi all,
> 
> this series removes the annoying struct block_device aliases, which can
> happen for a bunch of old floppy drivers (and z2ram).  In that case
> multiple struct block device instances for different dev_t's can point
> to the same gendisk, without being partitions.  The cause for that
> is the probe/get callback registered through blk_register_regions.
> 
> This series removes blk_register_region entirely, splitting it it into
> a simple xarray lookup of registered gendisks, and a probe callback
> stored in the major_names array that can be used for modprobe overrides
> or creating devices on demands when no gendisk is found.  The old
> remapping is gone entirely, and instead the 4 remaining drivers just
> register a gendisk for each operating mode.  In case of the two drivers
> that have lots of aliases that is done on-demand using the new probe
> callback, while for the other two I simply register all at probe time
> to keep things simple.
> 
> Note that the m68k drivers are compile tested only.
> 
> Changes since v3:
>  - keep kobj_map for char dev lookup for now, as the testbot found
>    some very strange and unexplained regressions, so I'll get back to
>    this later separately
>  - fix a commit message typo
> 
> Changes since v2:
>  - fix a wrong variable passed to ERR_PTR in the floppy driver
>  - slightly adjust the del_gendisk cleanups to prepare for the next
>    series touching this area
> 
> Changes since v1:
>  - add back a missing kobject_put in the cdev code
>  - improve the xarray delete loops
---end quoted text---