Re: [RFC PATCH v2 01/47] hugetlb: don't set PageUptodate for UFFDIO_CONTINUE

[Mike Kravetz]

On 11/21/22 10:33, James Houghton wrote:
> On Wed, Nov 16, 2022 at 8:30 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > On Fri, Oct 21, 2022 at 04:36:17PM +0000, James Houghton wrote:
> > > This is how it should have been to begin with. It would be very bad if
> > > we actually set PageUptodate with a UFFDIO_CONTINUE, as UFFDIO_CONTINUE
> > > doesn't actually set/update the contents of the page, so we would be
> > > exposing 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>
> > > ---
> > >  mm/hugetlb.c | 5 ++++-
> > >  1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 1a7dc7b2e16c..650761cdd2f6 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -6097,7 +6097,10 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > >        * preceding stores to the page contents become visible before
> > >        * the set_pte_at() write.
> > >        */
> > > -     __SetPageUptodate(page);
> > > +     if (!is_continue)
> > > +             __SetPageUptodate(page);
> > > +     else
> > > +             VM_WARN_ON_ONCE_PAGE(!PageUptodate(page), page);
> >
> > Yeah the old code looks wrong, I'm just wondering whether we can 100%
> > guarantee this for hugetlb.  E.g. for shmem that won't hold when we
> > uffd-continue on a not used page (e.g. by an over-sized fallocate()).
> >
> > Another safer approach is simply fail the ioctl if !uptodate, but if you're
> > certain then WARN_ON_ONCE sounds all good too.  At least I did have a quick
> > look on hugetlb fallocate() and pages will be uptodate immediately.
> 
> Failing the ioctl sounds better than only WARNing. I'll do that and
> drop the WARN_ON_ONCE for v1. Thanks!
> 

Sorry for the VERY late reply ...

After checking all the code paths, I do not think it is possible for a
!PageUptodate to be in the cache (target of continue).

ACK to failing the ioctl if not set, although I don't think it is possible
in current code.
-- 
Mike Kravetz