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

Peter Xu
2020-09-14T18:34:48Z

On Mon, Sep 14, 2020 at 10:32:11AM -0700, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 7:38 AM Jason Gunthorpe <jgg@nvidia.com> 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.

Sure, I'll try to prepare something like that and share it shortly.

Thanks,

--
Peter Xu