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

Jason Gunthorpe
2020-09-15T18:29:49Z

On Tue, Sep 15, 2020 at 01:05:53PM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 15, 2020 at 10:50:40AM -0400, Peter Xu wrote:
> > On Mon, Sep 14, 2020 at 08:28:51PM -0300, Jason Gunthorpe wrote:
> > > Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK
> > > together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy
> > > of the page and MADV_DONTFORK was needed to ensure that a future fork
> > > doesn't establish a COW that would break the DMA by moving the
> > > physical page over to the fork. DMA should stay with the process that
> > > called pin_user_pages_fast() (Is MADV_DONTFORK still needed with
> > > recent years work to GUP/etc? It is a pretty terrible ancient thing)
> >
> > ... Now I'm more confused on what has happened.
>
> I'm going to try to confirm that the MADV_DONTFORK is actually being
> done by userspace properly, more later.

It turns out the test is broken and does not call MADV_DONTFORK when
doing forks - it is an opt-in it didn't do.

It looks to me like this patch makes it much more likely that the COW
break after page pinning will end up moving the pinned physical page
to the fork while before it was not very common. Does that make sense?

Given that the tests are wrong it seems like broken userspace,
however, it also worked reliably for a fairly long time.

I'm waiting for confirmation that adding the missing MADV_DONTFORKS
restores all tests to success even with this patch.

Regards,
Jason