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 15:03:18 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jens Axboe <axboe@...nel.dk>, 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 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.

Logan