linux-kernel - Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

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Message-ID: <CAPcyv4h7mcFphv3j_vW-CGxXa2pjQgLqpBOLzHuTzAFzygai-Q@mail.gmail.com>
Date:   Thu, 1 Mar 2018 11:21:13 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     benh@....ibm.com
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-block@...r.kernel.org, Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        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>,
        Jérôme Glisse <jglisse@...hat.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Oliver OHalloran <oliveroh@....ibm.com>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory

On Wed, Feb 28, 2018 at 7:56 PM, Benjamin Herrenschmidt
<benh@....ibm.com> wrote:
> On Thu, 2018-03-01 at 14:54 +1100, Benjamin Herrenschmidt wrote:
>> On Wed, 2018-02-28 at 16:39 -0700, Logan Gunthorpe wrote:
>> > Hi Everyone,
>>
>>
>> So Oliver (CC) was having issues getting any of that to work for us.
>>
>> The problem is that acccording to him (I didn't double check the latest
>> patches) you effectively hotplug the PCIe memory into the system when
>> creating struct pages.
>>
>> This cannot possibly work for us. First we cannot map PCIe memory as
>> cachable. (Note that doing so is a bad idea if you are behind a PLX
>> switch anyway since you'd ahve to manage cache coherency in SW).
>
> Note: I think the above means it won't work behind a switch on x86
> either, will it ?

The devm_memremap_pages() infrastructure allows placing the memmap in
"System-RAM" even if the hotplugged range is in PCI space. So, even if
it is an issue on some configurations, it's just a simple adjustment
to where the memmap is placed.