System not booting since dm changes? (was Linux 4.20-rc1)

[Mike Snitzer]

On Mon, Nov 05 2018 at  5:25am -0500,
Michael Ellerman <mpe at ellerman.id.au> wrote:

> Linus Torvalds <torvalds at linux-foundation.org> writes:
> ...
> > Mike Snitzer (1):
> >     device mapper updates
> 
> Hi Mike,
> 
> Replying here because I can't find the device-mapper pull or the patch
> in question on LKML. I guess I should be subscribed to dm-devel.
> 
> We have a box that doesn't boot any more, bisect points at one of:
> 
>   cef6f55a9fb4 Mike Snitzer       dm table: require that request-based DM be layered on blk-mq devices 
>   953923c09fe8 Mike Snitzer       dm: rename DM_TYPE_MQ_REQUEST_BASED to DM_TYPE_REQUEST_BASED 
>   6a23e05c2fe3 Jens Axboe         dm: remove legacy request-based IO path 
> 
> 
> It's a Power8 system running Rawhide, it does have multipath, but I'm
> told it was setup by the Fedora installer, ie. nothing fancy.
> 
> The symptom is the system can't find its root filesystem and drops into
> the initramfs shell. The dmesg includes a bunch of errors like below:
> 
>   [   43.263460] localhost multipathd[1344]: sdb: fail to get serial
>   [   43.268762] localhost multipathd[1344]: mpatha: failed in domap for addition of new path sdb
>   [   43.268762] localhost multipathd[1344]: uevent trigger error
>   [   43.282065] localhost kernel: device-mapper: table: table load rejected: not all devices are blk-mq request-stackable
...
>
> Any ideas what's going wrong here?

"table load rejected: not all devices are blk-mq request-stackable"
speaks to the fact that you aren't using blk-mq for scsi (aka scsi-mq).

You need to use scsi_mod.use_blk_mq=Y on the kernel commandline (or set
CONFIG_SCSI_MQ_DEFAULT in your kernel config)

Mike