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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>,
        Christoph Hellwig <hch@lst.de>
Cc:     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,
        Stephen Bates <sbates@raithlin.com>,
        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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 <b5ab8fa3-34ac-3117-fa0d-5206c3939435@kernel.dk>
 <20180901082812.GB670@lst.de>
 <5f79c012-c6e1-56bb-62fd-0689181fb2c9@deltatee.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <59b28977-8f2a-6228-2050-03fae6bdbedd@kernel.dk>
Date:   Wed, 5 Sep 2018 13:26:51 -0600
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On 9/3/18 4:26 PM, Logan Gunthorpe wrote:
> 
> 
> On 01/09/18 02:28 AM, Christoph Hellwig wrote:
>> On Thu, Aug 30, 2018 at 01:11:18PM -0600, Jens Axboe wrote:
>>> 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.
>>
>> It is just a single branch, which will be predicted as not taken
>> for non-P2P users.  The benefit is that we get proper error checking
>> by doing it in the block code.
> 
> I personally agree with Christoph. But if there's consensus in the other
> direction or this is a real blocker moving this forward, I can remove it
> for the next version.

It's a simple branch because the check isn't exhaustive. It just checks
the first page. At that point you may as well just require the caller to
flag the bio/rq as being P2P, and then do a check for P2P compatibility
with the queue.

-- 
Jens Axboe