[PATCH v7 00/11] Speedup mremap on ppc64

[Linus Torvalds]

On Mon, Jun 7, 2021 at 3:10 AM Nick Piggin <npiggin at gmail.com> wrote:
>
> I'd really rather not do this, I'm not sure if micro benchmark captures everything.

I don't much care what powerpc code does _itnernally_ for this
architecture-specific mis-design issue, but I really don't want to see
more complex generic interfaces unless you have better hard numbers
for them.

So far the numbers are: "no observable difference".

It would have to be not just observable, but actually meaningful for
me to go "ok, we'll add this crazy flag that nobody else cares about".

And honestly, from everything I've seen on page table walker caches:
they are great, but once you start remapping big ranges and
invallidating megabytes of TLB's, the walker caches just aren't going
to be your issue.

But: numbers talk.  I'd take the sane generic interfaces as a first
cut. If somebody then has really compelling numbers, we can _then_
look at that "optimize for odd page table walker cache situation"
case.

And in the meantime, maybe you can talk to the hardware people and
tell them that you want the "flush range" capability to work right,
and that if the walker cache is <i>so</i> important they shouldn't
have made it a all-or-nothing flush.

            Linus