Re: dm cache: verify metadata device is exclussive

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Jul 24 2013 at  8:50pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> Do not allow a cache device to use a metadata device that is already
> in use by another cache device.
> 
> Add list member to the cache structure.  Also reorder members in the
> cache structure to eliminate 6 out of 7 holes (reclaiming 24 bytes).
> 
> Example:
>  echo "0 8192 linear 253:1 0" | dmsetup create metadata
>  echo "0 2097152 linear 253:1 8192" | dmsetup create ssd
>  echo "0 33554432 linear 253:0 0" | dmsetup create origin
>  echo "0 33554432 cache /dev/mapper/metadata /dev/mapper/ssd /dev/mapper/origin 512 0 default 0" | dmsetup create cache1
>  echo "0 33554432 cache /dev/mapper/metadata /dev/mapper/ssd /dev/mapper/origin 512 0 default 0" | dmsetup create cache2
> device-mapper: reload ioctl on cache2 failed: Device or resource busy
> Command failed
> 
> Kernel log shows:
> device-mapper: table: 253:8: cache: metadata device already in use by a cache
> device-mapper: ioctl: error adding target to table

Self-NACK.

This patch doesn't handle reloading a cache device.  The cache struct
member reshuffle may be useful to better align on cachelines and
eliminate holes but I'll repost a separate patch accordingly.