linux-kernel - Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory

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Date:   Thu, 1 Mar 2018 12:10:12 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Sagi Grimberg <sagi@...mberg.me>, 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
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        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>,
        Steve Wise <swise@...ngridcomputing.com>
Subject: Re: [PATCH v2 10/10] nvmet: Optionally use PCI P2P memory


> Wouldn't it all be simpler if the p2p_dev resolution would be private
> to the namespace?
> 
> So is adding some all the namespaces in a subsystem must comply to
> using p2p? Seems a little bit harsh if its not absolutely needed. Would
> be nice to export a subsystems between two ports (on two HCAs, across
> NUMA nodes) where the home node (primary path) would use p2p and
> failover would use host memory...
> 
> Can you help me understand why this is absolutely not feasible?

Yes, it would simplify things. However, as best as I can tell, when we 
allocate memory we don't know what namespace it will be used for. If 
there is a way, I could probably rework it so there's a P2P device per 
namespace.

Can you show me how we'd know the namespace to use in 
nvmet_rdma_map_sgl_keyed()?

Thanks,

Logan