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Date:   Mon, 17 Dec 2018 08:06:04 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        linux-block@vger.kernel.org, Jeff Moyer <jmoyer@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH 1/4] nvme-pci: correct mapping for poll queue(s)
Message-ID: <20181217070604.GA1694@lst.de>
References: <20181216022517.26650-1-ming.lei@redhat.com> <20181216022517.26650-2-ming.lei@redhat.com> <20181216161159.GB9957@lst.de> <20181217010051.GA1223@ming.t460p>
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On Mon, Dec 17, 2018 at 09:00:52AM +0800, Ming Lei wrote:
> On Sun, Dec 16, 2018 at 05:11:59PM +0100, Christoph Hellwig wrote:
> > On Sun, Dec 16, 2018 at 10:25:14AM +0800, Ming Lei wrote:
> > > If we don't have poll queue, its mapping should share default type's,
> > > instead of setting up one new mapping via blk_mq_map_queues().
> > 
> > No, if we don't have poll queues nr_maps should be 2.  I've already
> > posted a patch for that for nvme-pci which got it wrong (my fault..)
> 
> This patch doesn't touch nr_maps.
> 
> The issue is that if poll_queues/write_queues are zero, all three mapping
> should be same, however they aren't actually because we still use blk_mq_map_queues()
> to build a new mapping for poll queue.

In Jens' for-4.21 we only support polling with explicit poll queues,
and thus an explicit poll map.  Which means nr_maps must be 3 if you
support poll maps, and < 3 if you don't.  As said, the only user actually
got that wrong (my fault), but the fix is already queue up in the nvme
tree and on its way to Jens.