Re: [PATCH v2 01/46] hugetlb: don't set PageUptodate for UFFDIO_CONTINUE

[James Houghton]

On Tue, Feb 21, 2023 at 11:34 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 02/21/23 07:59, James Houghton wrote:
> > On Fri, Feb 17, 2023 at 4:42 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Feb 17, 2023 at 4:28 PM James Houghton <jthoughton@xxxxxxxxxx> wrote:
> > > >
> > > > If would be bad if we actually set PageUptodate with UFFDIO_CONTINUE;
> > > > PageUptodate indicates that the page has been zeroed, and we don't want
> > > > to give a non-zeroed page to the user.
> > > >
> > > > The reason this change is being made now is because UFFDIO_CONTINUEs on
> > > > subpages definitely shouldn't set this page flag on the head page.
> > > >
> > > > Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> > > >
> > > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > > index 07abcb6eb203..792cb2e67ce5 100644
> > > > --- a/mm/hugetlb.c
> > > > +++ b/mm/hugetlb.c
> > > > @@ -6256,7 +6256,16 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > > >          * preceding stores to the page contents become visible before
> > > >          * the set_pte_at() write.
> > > >          */
> > > > -       __folio_mark_uptodate(folio);
> > > > +       if (!is_continue)
> > > > +               __folio_mark_uptodate(folio);
> > > > +       else if (!folio_test_uptodate(folio)) {
> > > > +               /*
> > > > +                * This should never happen; HugeTLB pages are always Uptodate
> > > > +                * as soon as they are allocated.
> > > > +                */
> > >
> > > if (is_continue) then we grab a page from the page cache, no? Are
> > > pages in page caches always uptodate? Why? I guess that means they're
> > > mapped hence uptodate?
> > >
> > > Also this comment should explain why pages in the page cache are
> > > always uptodate, no? Because this error branch is hit if (is_continue
> > > && !folio_test_uptodate()), not when pages are freshly allocated.
> >
> > There was some discussion about it here[1].
> >
> > Without even thinking about how the pages become uptodate, I think
> > this patch is justified like this: UFFDIO_CONTINUE => we aren't
> > actually changing the contents of the page, so we shouldn't be
> > changing the uptodate-ness of the page.
>
> Agree!
>
> > HugeTLB pages in the page cache are always uptodate:
> > 1. fallocate -- the page is allocated, zeroed, marked as uptodate, and
> > then placed in the page cache.
> > 2. hugetlb_no_page -- same as above.
> >
> > So uptodate <=> "the page has been zeroed", so it would be very bad if
> > we gave a !uptodate page to userspace via UFFDIO_CONTINUE.
> >
> > I'll update the comment to something like:
> >
> > "HugeTLB pages are always Uptodate as soon as they are added to the
> > page cache. Given that we aren't changing the contents of the page, we
> > shouldn't be updating the Uptodate-ness of the page."
>
> Perhaps a better way of saying is that hugetlb pages are marked uptodate
> shortly after allocation when their contents are initialized.  Initialized
> data could be zero, or it could be contents copied from another location
> (such as in the UFFDIO_COPY case also handled in this routine).

I'll write something like this. Thank you!

>
> Saying "PageUptodate indicates that the page has been zeroed" as in the
> commit message is technically not correct.

And I'll make sure to update the commit description as well.

>
> Ack to the patch.

Thanks, Mike!