Re: [for-4.14 RFC PATCH 0/2] dm rq: eliminate historic blk-mq and .request_fn queue stacking restrictions

On Fri, Jul 14, 2017 at 10:02:06AM -0400, Mike Snitzer wrote:

On Fri, Jul 14 2017 at  3:12am -0400,
Christoph Hellwig [off-list ref] wrote:

quoted

Btw, we might want to expedite this to 4.13, a 4.13 now defaults
to blk-mq for scsi, and this patch would make sure that dm keeps
on just working with that switch.

Don't think we need to rush anything in response to that change in
SCSI's default.  old .request_fn DM multipath works happily ontop of
blk-mq devices (so long as all paths are blk-mq).

You're right.  In that case I think we should just skip this series
and I'll dust of the patch to just kill the non-mq support for 3.14
if the switch of scsi to default to mq works out for 3.13.

It is just blk-mq DM multipath ontop of old .request_fn paths that is
disallowed in current upstream code.

But again, I really don't see why we should even want/need to support
that mode... hence my question raised in this RFC.

I think this mode makes sense in the long run - to get rid of the
legacy request code in dm.  But as long as we keep both modes arounds
the use for it seems a big questionable indeed.