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

[peterz at infradead.org]

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.