Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache

[Barry Song]

On Thu, Feb 20, 2025 at 11:31 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Thu, Feb 20, 2025 at 12:25:19AM +1300, Barry Song wrote:
> > @@ -1079,9 +1080,19 @@ static int move_swap_pte(struct mm_struct *mm,
> >                        pmd_t *dst_pmd, pmd_t dst_pmdval,
> >                        spinlock_t *dst_ptl, spinlock_t *src_ptl)
> >  {
> > +     struct folio *folio;
> > +     swp_entry_t entry;
> > +
> >       if (!pte_swp_exclusive(orig_src_pte))
> >               return -EBUSY;
> >
> > +     entry = pte_to_swp_entry(orig_src_pte);
> > +     folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
>
> [Besides what's being discussed elsewhere..]
>
> swap_cache_get_folio() says:
>
>  * Caller must lock the swap device or hold a reference to keep it valid.
>
> Do we need get_swap_device() too here to avoid swapoff race?
>

Yep, thanks! Let me fix it in v2.

> > +     if (!IS_ERR(folio)) {
> > +             folio_put(folio);
> > +             return -EBUSY;
> > +     }
> > +
> >       double_pt_lock(dst_ptl, src_ptl);
> >
> >       if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte,
> > --
> > 2.39.3 (Apple Git-146)
> >
>
> --
> Peter Xu
>

Thanks
Barry