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

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Date:   Thu, 30 Aug 2018 13:19:25 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Logan Gunthorpe <logang@...tatee.com>,
        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
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        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 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