Re: [PATCH 05/16] dm: always defer request allocation to the owner of the request_queue

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 24, 2017 at 05:05:39AM -0500, Mike Snitzer wrote:
> possible and is welcomed cleanup.  The only concern I have is that using
> get_request() for the old request_fn request_queue eliminates the
> guaranteed availability of requests to allow for forward progress (on
> path failure or for the purposes of swap over mpath, etc).  This isn't a
> concern for blk-mq because as you know we have a fixed set of tags (and
> associated preallocated requests).
> 
> So I'm left unconvinced old request_fn request-based DM multipath isn't
> regressing in how request resubmission can be assured a request will be
> available when needed on retry in the face of path failure.

Mempool only need a size where we can make guaranteed requests, so for
get_request based drivers under dm the theoretical minimum size would be
one as we never rely on a second request to finish the first one,
and each request_queue has it's own mempool(s) to start with.

> dm_mod's 'reserved_rq_based_ios' module_param governs the minimum number
> of requests in the md->rq_pool (and defaults to 256 requests per
> request-based DM request_queue).  Whereas blk_init_rl()'s
> mempool_create_node() uses BLKDEV_MIN_RQ (4) yet q->nr_requests =
> BLKDEV_MAX_RQ (128).  Also, this patch eliminates the utility of
> 'reserved_rq_based_ios' module_param without actually removing it.

It's still used for the bio pool, so I couldn't simply remove it.

> Anyway, should blk-core evolve to allow drivers to specify a custom
> min_nr of requests in the old request_fn request_queue's mempool?  Or is
> my concern overblown?

Thanks to mempool_resize we could add that functionality.  I just don't
see an actual need for it.

> p.s. dm.c:dm_alloc_md_mempools() could be cleaned up a bit more since
> only bio-based DM will have a pools->io_pool moving forward; but I can
> circle back to that cleanup after.

If you're fine with doing more than the necessary changes in a patch
that needs to got into the block tree I'd be happy to apply my usual
cleanup magic to it.