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Subject: Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and
 check support for requests
To:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-nvme@lists.infradead.org, linux-rdma@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-block@vger.kernel.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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 <20180830185352.3369-8-logang@deltatee.com>
 <b5ab8fa3-34ac-3117-fa0d-5206c3939435@kernel.dk>
 <7c5b9d2c-d94e-582c-f1d7-0827dbb7df7c@deltatee.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <56e1a633-bad3-cac7-13b4-747f0001f5c8@kernel.dk>
Date:   Thu, 30 Aug 2018 13:19:25 -0600
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On 8/30/18 1:17 PM, Logan Gunthorpe wrote:
> 
> 
> On 30/08/18 01:11 PM, Jens Axboe wrote:
>> On 8/30/18 12:53 PM, Logan Gunthorpe wrote:
>>> QUEUE_FLAG_PCI_P2P is introduced meaning a driver's request queue
>>> supports targeting P2P memory.
>>>
>>> When a request is submitted we check if PCI P2PDMA memory is assigned
>>> to the first page in the bio. If it is, we ensure the queue it's
>>> submitted to supports it, and enforce REQ_NOMERGE.
>>
>> I think this belongs in the caller - both the validity check, and
>> passing in NOMERGE for this type of request. I don't want to impose
>> this overhead on everything, for a pretty niche case.
> 
> Well, the point was to prevent driver writers from doing the wrong
> thing. The WARN_ON would be a bit pointless in the driver if we rely on
> the driver to either do the right thing or add the WARN_ON themselves.
> 
> If I'm going to change anything I'd drop the warning entirely and move
> the NO_MERGE back into the caller...

Of course, if you move it into the caller, the warning makes no sense.

> Note: the check will be compiled out if the kernel does not support PCI P2P.

Sure, but then distros tend to enable everything...

-- 
Jens Axboe