Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

[Thomas Gleixner]

On Wed, Sep 23 2020 at 12:19, peterz wrote:
> On Mon, Sep 21, 2020 at 09:27:57PM +0200, Thomas Gleixner wrote:
>> Alternatively this could of course be solved with per CPU page tables
>> which will come around some day anyway I fear.
>
> Previously (with PTI) we looked at making the entire kernel map per-CPU,
> and that takes a 2K copy on switch_mm() (or more general, the user part
> of whatever the top level directory is for architectures that have a
> shared kernel/user page-table setup in the first place).
>
> The idea was having a fixed per-cpu kernel page-table, share a bunch of
> (kernel) page-tables between all CPUs and then copy in the user part on
> switch.
>
> I've forgotten what the plan was for ASID/PCID in that scheme.
>
> For x86_64 we've been fearing the performance of that 2k copy, but I
> don't think we've ever actually bit the bullet and implemented it to see
> how bad it really is.

I actually did at some point and depending on the workload the overhead
was clearly measurable. And yes, it fell apart with PCID and I could not
come up with a scheme for it which did not suck horribly. So I burried
the patches in the poison cabinet.

Aside of that, we'd need to implement that for a eight other
architectures as well...

Thanks,

        tglx