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Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI
 Memory
From:   Benjamin Herrenschmidt <benh@au1.ibm.com>
Reply-To: benh@au1.ibm.com
To:     Logan Gunthorpe <logang@deltatee.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Jens Axboe <axboe@kernel.dk>, Keith Busch <keith.busch@intel.com>,
        Oliver OHalloran <oliveroh@au1.ibm.com>,
        Alex Williamson <alex.williamson@redhat.com>,
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        =?ISO-8859-1?Q?J=E9r=F4me?= Glisse <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Christoph Hellwig <hch@lst.de>
Date:   Fri, 02 Mar 2018 10:25:34 +1100
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On Thu, 2018-03-01 at 16:19 -0700, Logan Gunthorpe wrote:
> 
> 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

The big problem is not the vmemmap, it's the linear mapping.

Cheers,
Ben.