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:   Wed, 5 Sep 2018 15:18:48 -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 9/5/18 3:03 PM, Logan Gunthorpe wrote:
> 
> 
> On 05/09/18 02:36 PM, Jens Axboe wrote:
>> On 9/5/18 2:32 PM, Logan Gunthorpe wrote:
>>>
>>>
>>> On 05/09/18 02:19 PM, Jens Axboe wrote:
>>>> On 9/5/18 2:18 PM, Logan Gunthorpe wrote:
>>>>>
>>>>>
>>>>> On 05/09/18 02:14 PM, Jens Axboe wrote:
>>>>>> But if the caller must absolutely know where the bio will end up, then
>>>>>> it seems super redundant. So I'd vote for killing this check, it buys
>>>>>> us absolutely nothing and isn't even exhaustive in its current form.
>>>>>
>>>>>
>>>>> Ok, I'll remove it for v6.
>>>>
>>>> Since the drivers needs to know it's doing it right, it might not
>>>> hurt to add a sanity check helper for that. Just have the driver
>>>> call it, and don't add it in the normal IO submission path.
>>>
>>> I'm not sure I really see the value in that. It's the same principle in
>>> asking the driver to do the WARN: if the developer knew enough to use
>>> the special helper, they probably knew well enough to do the rest correctly.
>>
>> I don't agree with that at all. It's a "is my request valid" helper,
>> it's not some obscure and rarely used functionality. You're making up
>> this API right now, if you really want it done for every IO, make it
>> part of the p2p submission process. You could even hide it behind a
>> debug thing, if you like.
> 
> There is no special p2p submission process. In the nvme-of case we are
> using the existing process and with the code in blk-core it didn't
> change it's process at all. Creating a helper will create one and I can
> look at making a pci_p2pdma_submit_bio() for v6; but if the developer
> screws up and still calls the regular submit_bio() things will only be
> very subtly broken and that won't be obvious.

I'm very sure that something that basic will be caught in review. I
don't care if you wrap the submission or just require the caller to
call some validity helper check first, fwiw.

And I think we're done beating the dead horse at this point.

-- 
Jens Axboe