Re: [PATCH 2/1] dm: do not allocate any mempools for blk-mq request-based DM

[Christoph Hellwig <hch@lst.de>]

On Mon, Apr 27, 2015 at 09:03:04PM -0400, Mike Snitzer wrote:
> Do not allocate the io_pool mempool for blk-mq request-based DM
> (DM_TYPE_MQ_REQUEST_BASED) in dm_alloc_rq_mempools().
> 
> Also refine __bind_mempools() to have more precise awareness of which
> mempools each type of DM device uses -- avoids mempool churn when
> reloading DM tables (particularly for DM_TYPE_REQUEST_BASED).

Btw, I looked into this code and didn't dare to touch it before I
understood how we deal with the case of dm-mpath using blk-mq and
low level driver not or the other way around.

As far as I can see we'd need the request mempool as long as the
low level driver does not use dm-mq, independent of what dm-mpath
itsel uses.

The code doesn't seem to handle this right, but as mentioned I'm not
100% sure and need to dive deeper into this.  Is there a place enforcing
dm-mpath is using the same request type as the underlying devices?