Re: [PATCH 05/28] mm, swap: sanitize swap cache lookup convention

[Barry Song]

On Tue, May 20, 2025 at 3:31 PM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Mon, May 19, 2025 at 12:38 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
> >
> > > From: Kairui Song <kasong@xxxxxxxxxxx>
> >
> > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > > index e5a0db7f3331..5b4f01aecf35 100644
> > > --- a/mm/userfaultfd.c
> > > +++ b/mm/userfaultfd.c
> > > @@ -1409,6 +1409,10 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> > >                               goto retry;
> > >                       }
> > >               }
> > > +             if (!folio_swap_contains(src_folio, entry)) {
> > > +                     err = -EBUSY;
> > > +                     goto out;
> > > +             }
> >
> > It seems we don't need this. In move_swap_pte(), we have been checking pte pages
> > are stable:
> >
> >         if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> >                                  dst_pmd, dst_pmdval)) {
> >                 double_pt_unlock(dst_ptl, src_ptl);
> >                 return -EAGAIN;
> >         }
>
> The tricky part is when swap_cache_get_folio returns the folio, both
> folio and ptes are unlocked. So is it possible that someone else
> swapped in the entries, then swapped them out again using the same
> entries?
>
> The folio will be different here but PTEs are still the same value to
> they will pass the is_pte_pages_stable check, we previously saw
> similar races with anon fault or shmem. I think more strict checking
> won't hurt here.

This doesn't seem to be the same case as the one you fixed in
do_swap_page(). Here, we're hitting the swap cache, whereas in that
case, there was no one hitting the swap cache, and you used
swap_prepare() to set up the cache to fix the issue.

By the way, if we're not hitting the swap cache, src_folio will be
NULL. Also, it seems that folio_swap_contains(src_folio, entry) does
not guard against that case either.

But I suspect we won't have a problem, since we're not swapping in —
we didn't read any stale data, right? Swap-in will only occur after we
move the PTEs.

>
> >
> > Also, -EBUSY is somehow incorrect error code.
>
> Yes, thanks, I'll use EAGAIN here just like move_swap_pte.
>
>
> >
> > >               err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte,
> > >                               orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval,
> > >                               dst_ptl, src_ptl, src_folio);
> > >
> >

Thanks
Barry