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Date:   Mon, 17 Dec 2018 11:57:25 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>, Jeff Moyer <jmoyer@redhat.com>,
        Mike Snitzer <snitzer@redhat.com>
Subject: Re: [PATCH V2 2/4] blk-mq: fix shared queue mapping
Message-ID: <20181217105725.GA5826@lst.de>
References: <20181217104248.5828-1-ming.lei@redhat.com> <20181217104248.5828-3-ming.lei@redhat.com>
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On Mon, Dec 17, 2018 at 06:42:46PM +0800, Ming Lei wrote:
> From: Christoph Hellwig <hch@lst.de>
> 
> Even though poll_queues are zero, nvme's mapping for HCTX_TYPE_POLL still
> may be setup via blk_mq_map_queues() which cause different mapping compared
> with HCTX_TYPE_DEFAULT's mapping built from managed irq affinity.
> 
> This mapping will cause hctx->type to be over-written in blk_mq_map_swqueue(),
> then the whole mapping may become broken, for example, one same ctx can be
> mapped to different hctxs with same hctx type. This bad mapping has caused
> IO hang in simple dd test, as reported by Mike.
> 
> This patch sets map->nr_queues as zero explictly if there is zero
> queues for such queue type, also maps to correct hctx if .nr_queues of the
> queue type is zero.
> 
> Cc: Jeff Moyer <jmoyer@redhat.com>
> Cc: Mike Snitzer <snitzer@redhat.com>
> Cc: Christoph Hellwig <hch@lst.de>
> 
> (don't handle zero .nr_queues map in blk_mq_map_swqueue())
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

I actually think we should split this into three patches:

 1) skip zero-queue maps in blk_mq_map_swqueue
 2) skip zero-queue maps in blk_mq_map_queue
 3) don't duplicate maps in nvme

sorry for misleading you with my quick WIP patch.

I can send send patches for my two with a proper changelogs, but
I'll leave blk_mq_map_swqueue to you as I don't full understand
the rationale, even if it looks sensible to me.