Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[Linus Torvalds]

On Mon, Sep 14, 2020 at 7:38 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> I don't have a detailed explanation right now, but this patch appears
> to be causing a regression where RDMA subsystem tests fail. Tests
> return to normal when this patch is reverted.
>
> It kind of looks like the process is not seeing DMA'd data to a
> pin_user_pages()?

I'm a nincompoop. I actually _talked_ to Hugh Dickins about this when
he raised concerns, and I dismissed his concerns with "but PAGE_PIN is
special".

As usual, Hugh was right. Page pinning certainly _is_ special, but
it's not that different from the regular GUP code.

But in the meantime, I have a lovely confirmation from the kernel test
robot, saying that commit 09854ba94c results in a
"vm-scalability.throughput 31.4% improvement", which was what I was
hoping for - the complexity wasn't just complexity, it was active
badness due to the page locking horrors.

I think what we want to do is basically do the "early COW", but only
do it for FOLL_PIN (and not turn them into writes for anything but the
COW code). So basically redo the "enforced COW mechanism", but rather
than do it for everything, now do it only for FOLL_PIN, and only in
that COW path.

Peter - any chance you can look at this? I'm still looking at the page
lock fairness performance regression, although I now think I have a
test patch for Phoronix to test out.

                     Linus