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

Benjamin Herrenschmidt
2018-03-01T23:00:04Z

On Thu, 2018-03-01 at 14:57 -0700, Logan Gunthorpe wrote:
>
> On 01/03/18 02:45 PM, Logan Gunthorpe wrote:
> > It handles it fine for many situations. But when you try to map
> > something that is at the end of the physical address space then the
> > spares-vmemmap needs virtual address space that's the size of the
> > physical address space divided by PAGE_SIZE which may be a little bit
> > too large...
>
> Though, considering this more, maybe this shouldn't be a problem...
>
> Lets say you have 56bits of address space.

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.

> So I'm not sure what the problem with this is.
>
> We still have to ensure all the arches map the memory with the right
> cache bits but that should be relatively easy to solve.
>
> Logan