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Subject: Re: [bug] __blk_mq_run_hw_queue suspicious rcu usage
To:     David Rientjes <rientjes@google.com>,
        Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@kernel.org>
Cc:     Tom Lendacky <thomas.lendacky@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Ming Lei <ming.lei@redhat.com>,
        Peter Gonda <pgonda@google.com>,
        Jianxiong Gao <jxgao@google.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, iommu@lists.linux-foundation.org
References: <alpine.DEB.2.21.1909041434580.160038@chino.kir.corp.google.com>
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 <alpine.DEB.2.21.1909161641320.9200@chino.kir.corp.google.com>
 <alpine.DEB.2.21.1909171121300.151243@chino.kir.corp.google.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <4cd700ef-f2fc-2bb5-35d2-0b2194accc45@kernel.dk>
Date:   Tue, 17 Sep 2019 12:32:45 -0600
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On 9/17/19 12:23 PM, David Rientjes wrote:
> On Mon, 16 Sep 2019, David Rientjes wrote:
> 
>> Brijesh and Tom, we currently hit this any time we boot an SEV enabled
>> Ubuntu 18.04 guest; I assume that guest kernels, especially those of such
>> major distributions, are expected to work with warnings and BUGs when
>> certain drivers are enabled.
>>
>> If the vmap purge lock is to remain a mutex (any other reason that
>> unmapping aliases can block?) then it appears that allocating a dmapool
>> is the only alternative.  Is this something that you'll be addressing
>> generically or do we need to get buy-in from the maintainers of this
>> specific driver?
>>
> 
> We've found that the following applied on top of 5.2.14 suppresses the
> warnings.
> 
> Christoph, Keith, Jens, is this something that we could do for the nvme
> driver?  I'll happily propose it formally if it would be acceptable.

No, this is not going to be acceptable, I'm afraid. This tells blk-mq
that the driver always needs blocking context for queueing IO, which
will increase latencies for the cases where we'd otherwise issue IO
directly from the context that queues it.

-- 
Jens Axboe