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

Benjamin Herrenschmidt
2018-03-01T23:25:34Z

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.