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Date:   Fri, 15 Jul 2022 13:39:08 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Axel Rasmussen <axelrasmussen@google.com>
Cc:     Miaohe Lin <linmiaohe@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in
 hugetlb_mcopy_atomic_pte
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On Fri, Jul 15, 2022 at 10:28:44AM -0700, Axel Rasmussen wrote:
> On Fri, Jul 15, 2022 at 10:07 AM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Fri, Jul 15, 2022 at 09:45:37AM -0700, Axel Rasmussen wrote:
> > > I agree we should either:
> > > - Update the UFFD selftest to exercise this case
> > > - Or, don't allow it, update vma_can_userfault() to also require VM_SHARED
> > > for VM_UFFD_MINOR registration.
> > >
> > > The first one is unfortunately not completely straightforward as Peter
> > > described. I would say it's probably not worth holding up this fix while we
> > > wait for it to happen?
> >
> > Agreed, Andrew has already queued it.  It actually is a real fix since we
> > never forbid the user running private mappings upon minor faults, so
> > it's literally a bug in kernel irrelevant of the kselftest.
> >
> > >
> > > I don't really have a strong preference between the two. The second option
> > > is what I originally proposed in the first version of the minor fault
> > > series, so going back to that isn't a problem at least from my perspective.
> > > If in the future we find a real use case for this, we could always easily
> > > re-enable it and add selftests for it at that point.
> >
> > I'd go for fixing the test case if possible.  Mike, would it be fine if we
> > go back to /dev/hugepages path based approach in the test case?
> 
> One possible alternative, can we use memfd_create() with MFD_HUGE_*?
> This afaict lets us have an fd so we can create two mappings,
> without having to mount hugetlbfs, pass in a path to the test, ...

Sounds good. :) We can also rework the shared hugetlb too.  Wanna post a
patch?  I can do that too, let me know otherwise.  Thanks!

-- 
Peter Xu