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

[Peter Xu]

 On Mon, Sep 14, 2020 at 02:34:36PM -0400, Peter Xu wrote:
> 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@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.
> 
> Sure, I'll try to prepare something like that and share it shortly.

Jason, would you please try the attached patch to see whether it unbreaks the
rdma test?  Thanks!

-- 
Peter Xu