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Date:   Thu, 29 Apr 2021 11:46:18 +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>,
        Keith Busch <kbusch@kernel.org>,
        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: <20210429094618.GA15311@lst.de>
References: <20210427161619.1294399-1-hch@lst.de> <20210427161619.1294399-15-hch@lst.de> <YIjJRiyA26gELV+d@T590> <20210429072737.GA3873@lst.de> <YIpiS0d1NeoX6p0H@T590>
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On Thu, Apr 29, 2021 at 03:37:47PM +0800, Ming Lei wrote:
> > > 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.
> 
> BLK_MQ_F_BLOCKING can be called from submitting context, see
> blk_mq_try_issue_directly().

The all drivers setting it have a problem, as the blk-mq core sets
BLK_MQ_F_BLOCKING for all of them.  The right fix would be to not make
it block for REQ_NOWAIT I/O..