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

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Message-ID: <595acefb-18fc-e650-e172-bae271263c4c@deltatee.com>
Date:   Thu, 1 Mar 2018 16:19:59 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     benh@....ibm.com, Dan Williams <dan.j.williams@...el.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Keith Busch <keith.busch@...el.com>,
        Oliver OHalloran <oliveroh@....ibm.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-rdma <linux-rdma@...r.kernel.org>,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org, linux-block@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory



On 01/03/18 04:00 PM, Benjamin Herrenschmidt wrote:
> We use only 52 in practice but yes.
> 
>>   That's 64PB. If you use need
>> a sparse vmemmap for the entire space it will take 16TB which leaves you
>> with 63.98PB of address space left. (Similar calculations for other
>> numbers of address bits.)
> 
> We only have 52 bits of virtual space for the kernel with the radix
> MMU.

Ok, assuming you only have 52 bits of physical address space: the sparse 
vmemmap takes 1TB and you're left with 3.9PB of address space for other 
things. So, again, why doesn't that work? Is my math wrong?

Logan