linux-kernel - Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and check support for requests

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Message-ID: <59b28977-8f2a-6228-2050-03fae6bdbedd@kernel.dk>
Date:   Wed, 5 Sep 2018 13:26:51 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and
 check support for requests

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