Re: [BUG]userfaultfd_move fails to move a folio when swap-in occurs concurrently with swap-out

[Barry Song <21cnbao@gmail.com>]

On Tue, May 27, 2025 at 12:39 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 23.05.25 01:23, Barry Song wrote:
> > Hi All,
>
> Hi!
>
> >
> > I'm encountering another bug that can be easily reproduced using the small
> > program below[1], which performs swap-out and swap-in in parallel.
> >
> > The issue occurs when a folio is being swapped out while it is accessed
> > concurrently. In this case, do_swap_page() handles the access. However,
> > because the folio is under writeback, do_swap_page() completely removes
> > its exclusive attribute.
> >
> > do_swap_page:
> >                 } else if (exclusive && folio_test_writeback(folio) &&
> >                            data_race(si->flags & SWP_STABLE_WRITES)) {
> >                          ...
> >                          exclusive = false;
> >
> > As a result, userfaultfd_move() will return -EBUSY, even though the
> > folio is not shared and is in fact exclusively owned.
> >
> >                          folio = vm_normal_folio(src_vma, src_addr,
> > orig_src_pte);
> >                          if (!folio || !PageAnonExclusive(&folio->page)) {
> >                                  spin_unlock(src_ptl);
> > +                               pr_err("%s %d folio:%lx exclusive:%d
> > swapcache:%d\n",
> > +                                       __func__, __LINE__, folio,
> > PageAnonExclusive(&folio->page),
> > +                                       folio_test_swapcache(folio));
> >                                  err = -EBUSY;
> >                                  goto out;
> >                          }
> >
> > I understand that shared folios should not be moved. However, in this
> > case, the folio is not shared, yet its exclusive flag is not set.
> >
> > Therefore, I believe PageAnonExclusive is not a reliable indicator of
> > whether a folio is truly exclusive to a process.
>
> It is. The flag *not* being set is not a reliable indicator whether it
> is really shared. ;)
>
> The reason why we have this PAE workaround (dropping the flag) in place
> is because the page must not be written to (SWP_STABLE_WRITES). CoW
> reuse is not possible.
>
> uffd moving that page -- and in that same process setting it writable,
> see move_present_pte()->pte_mkwrite() -- would be very bad.

An alternative approach is to make the folio writable only when we are
reasonably certain it is exclusive; otherwise, it remains read-only. If the
destination is later written to and the folio has become exclusive, it can
be reused directly. If not, a copy-on-write will occur on the destination
address, transparently to userspace. This avoids Lokesh’s userspace-based
strategy, which requires forcing a write to the source address.

>
> >
> > The kernel log output is shown below:
> > [   23.009516] move_pages_pte 1285 folio:fffffdffc01bba40 exclusive:0
> > swapcache:1
> >
> > I'm still struggling to find a real fix; it seems quite challenging.
>
> PAE tells you that you can immediately write to that page without going
> through CoW. However, here, CoW is required.
>
> > Please let me know if you have any ideas. In any case It seems
> > userspace should fall back to userfaultfd_copy.
>
> We could try detecting whether the page is now exclusive, to reset PAE.
> That will only be possible after writeback completed, so it adds
> complexity without being able to move the page in all cases (during
> writeback).
>
> Letting userspace deal with that in these rate scenarios is
> significantly easier.

Right, this appears to introduce the least change—essentially none—to the
kernel, while shifting more noise to userspace :-)

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry