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Date:   Thu, 29 Apr 2021 09:27:37 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Jeffle Xu <jefflexu@linux.alibaba.com>,
        Damien Le Moal <Damien.LeMoal@wdc.com>,
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        Sagi Grimberg <sagi@grimberg.me>,
        "Wunderlich, Mark" <mark.wunderlich@intel.com>,
        "Vasudevan, Anil" <anil.vasudevan@intel.com>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 14/15] nvme-multipath: set QUEUE_FLAG_NOWAIT
Message-ID: <20210429072737.GA3873@lst.de>
References: <20210427161619.1294399-1-hch@lst.de> <20210427161619.1294399-15-hch@lst.de> <YIjJRiyA26gELV+d@T590>
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On Wed, Apr 28, 2021 at 10:32:38AM +0800, Ming Lei wrote:
> > diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
> > index 4e2c3a6787e9..1d17b2387884 100644
> > --- a/drivers/nvme/host/multipath.c
> > +++ b/drivers/nvme/host/multipath.c
> > @@ -442,6 +442,8 @@ int nvme_mpath_alloc_disk(struct nvme_ctrl *ctrl, struct nvme_ns_head *head)
> >  	if (!q)
> >  		goto out;
> >  	blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
> > +	blk_queue_flag_set(QUEUE_FLAG_NOWAIT, q);
> 
> BLK_MQ_F_BLOCKING is set for nvme-tcp, and the blocking may be done inside
> nvme_ns_head_submit_bio(), is that one problem?

The point of BLK_MQ_F_BLOCKING is that ->queue_rq can block, and
because of that it is not called from the submitting context but in
a work queue.  nvme-tcp also sets QUEUE_FLAG_NOWAIT, just like all blk-mq
drivers.