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:   Mon, 10 Sep 2018 12:11:01 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, 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 10/09/18 10:41 AM, Christoph Hellwig wrote:
> On Wed, Sep 05, 2018 at 03:03:18PM -0600, Logan Gunthorpe wrote:
>> 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 just saw you added that "helper" in your tree.  Please don't, it is
> a negative value add as it doesn't help anything with the checking.

Alright, so what's the consensus then? Just have a check in
nvmet_bdev_execute_rw() to add REQ_NOMERGE when appropriate? Jens is
pretty dead set against adding to the common path.

Logan


P.S. Here's the commit in question for anyone else on the list:

https://github.com/sbates130272/linux-p2pmem/commit/eeabe0bc94491d3eec4fe872274a9e3b4cdea538