linux-next: possible bug in 'block: remove the BIO_NULL_MAPPED flag'

[Marc Hartmayer]

Hi Christoph, Jens,

I found an interesting bug in my KVM guest (tested on s390x). The guest
uses a virtio-scsi disk and the current linux-next kernel. The problem
is that I cannot get the SCSI ID of the attached SCSI disk. Running the
command `lsscsi --scsi_id` in the guest returns:

root@qemus390x:~# lsscsi --scsi_id
[0:0:0:0]    disk    Linux    scsi_debug       0190  /dev/sda   -

but the expected result is something like:

root@qemus390x:~# lsscsi --scsi_id
[0:0:0:0]    disk    Linux    scsi_debug       0190  /dev/sda   33333333000002710

Also there is no /dev/disk/by-id/scsi-* path created. I bisected the
problem to...

commit f3256075ba49d80835b601bfbff350a2140b2924 (HEAD, refs/bisect/bad)
Author: Christoph Hellwig <hch@xxxxxx>
Date:   Thu Aug 27 17:37:45 2020 +0200

    block: remove the BIO_NULL_MAPPED flag

When I reverted this commit the problem was gone. Any ideas what the
problem is? Thanks in advance.

Best regards,
 Marc