Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

Peter Xu
2020-09-23T13:50:15Z

On Wed, Sep 23, 2020 at 11:22:05AM +0200, Jan Kara wrote:
> On Tue 22-09-20 13:01:13, John Hubbard wrote:
> > On 9/22/20 3:33 AM, Jan Kara wrote:
> > > On Mon 21-09-20 23:41:16, John Hubbard wrote:
> > > > On 9/21/20 2:20 PM, Peter Xu wrote:
> > > > ...
> > > > > + if (unlikely(READ_ONCE(src_mm->has_pinned) &&
> > > > > + page_maybe_dma_pinned(src_page))) {
> > > >
> > > > This condition would make a good static inline function. It's used in 3
> > > > places, and the condition is quite special and worth documenting, and
> > > > having a separate function helps with that, because the function name
> > > > adds to the story. I'd suggest approximately:
> > > >
> > > > page_likely_dma_pinned()
> > > >
> > > > for the name.
> > >
> > > Well, but we should also capture that this really only works for anonymous
> > > pages. For file pages mm->has_pinned does not work because the page may be
> > > still pinned by completely unrelated process as Jann already properly
> > > pointed out earlier in the thread. So maybe anon_page_likely_pinned()?
> > > Possibly also assert PageAnon(page) in it if we want to be paranoid...
> > >
> > > Honza
> >
> > The file-backed case doesn't really change anything, though:
> > page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you
> > can get a false positive. Just like here, with an mm->has_pinned that
> > could be a false positive for a process.
> >
> > And for that reason, I'm also not sure an "assert PageAnon(page)" is
> > desirable. That assertion would prevent file-backed callers from being
> > able to call a function that provides a fuzzy answer, but I don't see
> > why you'd want or need to do that. The goal here is to make the fuzzy
> > answer a little bit more definite, but it's not "broken" just because
> > the result is still fuzzy, right?
> >
> > Apologies if I'm missing a huge point here... :)
>
> But the problem is that if you apply mm->has_pinned check on file pages,
> you can get false negatives now. And that's not acceptable...

Do you mean the case where proc A pinned page P from a file, then proc B mapped
the same page P on the file, then fork() on proc B?

If proc B didn't explicitly pinned page P in B's address space too, shouldn't
we return "false" for page_likely_dma_pinned(P)? Because if proc B didn't pin
the page in its own address space, I'd think it's ok to get the page replaced
at any time as long as the content keeps the same. Or couldn't we?

Thanks,

--
Peter Xu