Re: [patch V163 27/51] x86/mm/pti: Populate user PGD

From: Peter Zijlstra
Date: Mon Dec 18 2017 - 15:58:12 EST

On Mon, Dec 18, 2017 at 12:45:13PM -0800, Dave Hansen wrote:
> On 12/18/2017 12:41 PM, Peter Zijlstra wrote:
> >> I also don't think the user_shared area of the fixmap can get *that*
> >> big. Does anybody know offhand what the theoretical limits are there?
> > Problem there is the nr_cpus term I think, we currently have up to 8k
> > CPUs, but I can see that getting bigger in the future.
> It only matters if we go over 512GB, though. Is the per-cpu part of the
> fixmap ever more than 512GB/8k=64MB?

Unlikely, I think the LDT (@ 32 pages / 128K) and the DS (@ 2*4 pages /
32K) are the largest entries in there.