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

Peter Xu
2020-09-24T18:49:45Z

On Wed, Sep 23, 2020 at 01:19:08PM -0700, John Hubbard wrote:
> On 9/23/20 8:44 AM, Peter Xu wrote:
> > On Wed, Sep 23, 2020 at 04:01:14PM +0200, Jan Kara wrote:
> > > On Wed 23-09-20 09:50:04, Peter Xu wrote:
> ...
> > > > > 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?
> > >
> > > Yes.
>
> aha, thanks for spelling out the false negative problem.
>
> > >
> > > > 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?
> > >
> > > So it depends on the reason why you call page_likely_dma_pinned(). For your
> > > COW purposes the check is correct but e.g. for "can filesystem safely
> > > writeback this page" the page_likely_dma_pinned() would be wrong. So I'm
> > > not objecting to the mechanism as such. I'm mainly objecting to the generic
> > > function name which suggests something else than what it really checks and
> > > thus it could be used in wrong places in the future... That's why I'd
> > > prefer to restrict the function to PageAnon pages where there's no risk of
> > > confusion what the check actually does.
> >
> > How about I introduce the helper as John suggested, but rename it to
> >
> > page_maybe_dma_pinned_by_mm()
> >
> > ?
> >
> > Then we also don't need to judge on which is more likely to happen (between
> > "maybe" and "likely", since that will confuse me if I only read these words..).
> >
>
> You're right, it is too subtle of a distinction after all. I agree that sticking
> with "_maybe_" avoids that confusion.
>
>
> > I didn't use any extra suffix like "cow" because I think it might be useful for
> > things besides cow. Fundamentally the new helper will be mm-based, so "by_mm"
> > seems to suite better to me.
> >
> > Does that sound ok?
> >
>
> Actually, Jan nailed it. I just wasn't understanding his scenario, but now that
> I do, and considering your other point about wording, I think we end up with:
>
> anon_page_maybe_pinned()
>
> as a pretty good name for a helper function. (We don't want "_mm" because that
> refers more to the mechanism used internally, rather than the behavior of the
> function. "anon_" adds more meaning.)

Actually it was really my intention when I suggested "_by_mm", because IMHO the
new helper actually means "whether this page may be pinned by _this mm_ (not
any other address space)". IOW, the case that Jan mentioned on the share page
can be reflected in this case, because although that page was pinned, however
it was not pinned "by this mm" for e.g. proc B above.

Though I've no strong opinion either. I'll start with anon_page_maybe_pinned().
To me it's probably more important to prepare the next spin first and see
whether we'd still like it for this release.

Thanks,

--
Peter Xu