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

From: Logan Gunthorpe <logang@deltatee.com>
To: Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>
Cc: "Alex Williamson" <alex.williamson@redhat.com>,
	linux-nvdimm@lists.01.org, linux-rdma@vger.kernel.org,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvme@lists.infradead.org, linux-block@vger.kernel.org,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Jason Gunthorpe" <jgg@mellanox.com>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Max Gurtovoy" <maxg@mellanox.com>,
	"Christian König" <christian.koenig@amd.com>
Subject: Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and check support for requests
Date: Wed, 5 Sep 2018 15:03:18 -0600	[thread overview]
Message-ID: <c41c802c-9235-5383-2dfa-af9c1553f11b@deltatee.com> (raw)
In-Reply-To: <34d9b2f7-9e6d-4a0a-77e5-ec0e99610f5c@kernel.dk>

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
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