Re: dm-mpath: Work with blk multi-queue drivers

[Christoph Hellwig <hch@infradead.org>]

On Thu, Sep 25, 2014 at 09:57:41AM -0600, Keith Busch wrote:
> Thank you for all the background information. This definitely gives me
> a lot more to think about.
> 
> For my part, the goal was to change as little as possible to get basic
> blk-mq support working safely without regressing, and performance is
> not even on my radar yet. I purposefully did not try to understand the
> existing design well enough to propose re-arching. If we can address the
> 'request' life cycle management duality issue, would this be acceptable
> as a stopgap for blk-mq support?

I fully agree with going for the stop gap for now.  I tried going the
long way when I gave it a try and got stuck.

If people believe the get_request in the map path is harmful for the old
code we might have to make your change conditional just for blk-mq.  For
blk-mq request allocation never dips into the general purpose memory
pool, so it should be fine for that case.