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From:   Logan Gunthorpe <logang@deltatee.com>
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Date:   Thu, 1 Mar 2018 16:19:59 -0700
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Subject: Re: [PATCH v2 00/10] Copy Offload in NVMe Fabrics with P2P PCI Memory
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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