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

From: Dave Hansen
Date: Mon Dec 18 2017 - 15:45:26 EST

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?

In any case, it's not hard to make a little loop. I think the common
case, at least is that we only do 4 PGDs.