Re: [PATCH RFC] mm: entirely reuse the whole anon mTHP in do_wp_page

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

On Sat, Aug 31, 2024 at 9:59 PM David Hildenbrand <david@redhat.com> wrote:
>
>
> > +             idx = folio_page_idx(folio, vmf->page);
> > +             folio_start = address - idx * PAGE_SIZE;
> > +             folio_end = folio_start + nr * PAGE_SIZE;
> > +
> > +             if (unlikely(folio_start < max(address & PMD_MASK, vma->vm_start)))
> > +                     return false;
> > +             if (unlikely(folio_end > pmd_addr_end(address, vma->vm_end)))
> > +                     return false;
> > +             folio_ptep = vmf->pte - idx;
> > +             folio_pte = ptep_get(folio_ptep);
> > +             if (!pte_present(folio_pte) || pte_pfn(folio_pte) != folio_pfn(folio))
> > +                     return false;
> > +             if (folio_pte_batch(folio, folio_start, folio_ptep, folio_pte, nr, 0,
> > +                             NULL, NULL, NULL) != nr)
> > +                     return false;
> > +             if (folio_mapcount(folio) != nr)
> > +                     return false;
>
> BTW, you're not checking against the refcount (and it's all a bit racy
> on concurrent unmapping!). So you're re-introducing the vmsplice
> child->parent attak.

i don't quite understand this, you mean the below is not enough?

-       if (folio_test_ksm(folio) || folio_ref_count(folio) > 3)
+       if (folio_test_ksm(folio) || folio_ref_count(folio) > 2 + nr)
                return false;
        if (!folio_test_lru(folio))
                /*
@@ -3591,13 +3627,13 @@ static bool wp_can_reuse_anon_folio(struct folio *folio,
                 * remote LRU caches or references to LRU folios.
                 */
                lru_add_drain();
-       if (folio_ref_count(folio) > 1 + folio_test_swapcache(folio))
+       if (folio_ref_count(folio) > nr + folio_test_swapcache(folio))
                return false;
        if (!folio_trylock(folio))
                return false;
        if (folio_test_swapcache(folio))
                folio_free_swap(folio);
-       if (folio_test_ksm(folio) || folio_ref_count(folio) != 1) {
+       if (folio_test_ksm(folio) || folio_ref_count(folio) != nr) {
                folio_unlock(folio);
                return false;

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