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

[Jason Gunthorpe]

On Mon, Sep 14, 2020 at 05:15:15PM -0400, Peter Xu wrote:
> 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!

Sure, I'll get back to you

> Fast gup is not affected by this because it is never used with FOLL_PIN.

? What is pin_user_pages_fast() then? That is the API the failing test
is using.

> Note: hugetlbfs is not considered throughout this patch, because it's missing
> some required bits after all (like proper setting of FOLL_COW when page fault
> retries).  Considering we may want to unbreak RDMA tests even during the rcs,
> this patch only fixes the non-hugetlbfs cases. THPs should still be in count.

People do RDMA with hugetlbfs too.

Just as an aside, the RDMA stuff is also supposed to set MADV_DONTFORK
on these regions, so I'm a bit puzzled what is happening here.

Jason