Re: [PATCH v2 4/5] mm: swap: entirely map large folios found in swapcache

[Barry Song]

On Tue, Apr 16, 2024 at 2:27 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
>
> Added Khalid for arch_do_swap_page().
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > On Mon, Apr 15, 2024 at 8:39 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> >>
> >> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> [snip]
>
> >>
> >> > +     bool any_swap_shared = false;
> >> >
> >> >       if (!pte_unmap_same(vmf))
> >> >               goto out;
> >> > @@ -4137,6 +4141,35 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >> >        */
> >> >       vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
> >> >                       &vmf->ptl);
> >>
> >> We should move pte check here.  That is,
> >>
> >>         if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> >>                 goto out_nomap;
> >>
> >> This will simplify the situation for large folio.
> >
> > the plan is moving the whole code block
> >
> > if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio))
> >
> > after
> >         if (unlikely(!folio_test_uptodate(folio))) {
> >                 ret = VM_FAULT_SIGBUS;
> >                 goto out_nomap;
> >         }
> >
> > though we couldn't be !folio_test_uptodate(folio)) for hitting
> > swapcache but it seems
> > logically better for future use.
>
> LGTM, Thanks!
>
> >>
> >> > +
> >> > +     /* We hit large folios in swapcache */
> >>
> >> The comments seems unnecessary because the code tells that already.
> >>
> >> > +     if (start_pte && folio_test_large(folio) && folio_test_swapcache(folio)) {
> >> > +             int nr = folio_nr_pages(folio);
> >> > +             int idx = folio_page_idx(folio, page);
> >> > +             unsigned long folio_start = vmf->address - idx * PAGE_SIZE;
> >> > +             unsigned long folio_end = folio_start + nr * PAGE_SIZE;
> >> > +             pte_t *folio_ptep;
> >> > +             pte_t folio_pte;
> >> > +
> >> > +             if (unlikely(folio_start < max(vmf->address & PMD_MASK, vma->vm_start)))
> >> > +                     goto check_pte;
> >> > +             if (unlikely(folio_end > pmd_addr_end(vmf->address, vma->vm_end)))
> >> > +                     goto check_pte;
> >> > +
> >> > +             folio_ptep = vmf->pte - idx;
> >> > +             folio_pte = ptep_get(folio_ptep);
> >>
> >> It's better to construct pte based on fault PTE via generalizing
> >> pte_next_swp_offset() (may be pte_move_swp_offset()).  Then we can find
> >> inconsistent PTEs quicker.
> >
> > it seems your point is getting the pte of page0 by pte_next_swp_offset()
> > unfortunately pte_next_swp_offset can't go back. on the other hand,
> > we have to check the real pte value of the 0nd entry right now because
> > swap_pte_batch() only really reads pte from the 1st entry. it assumes
> > pte argument is the real value for the 0nd pte entry.
> >
> > static inline int swap_pte_batch(pte_t *start_ptep, int max_nr, pte_t pte)
> > {
> >         pte_t expected_pte = pte_next_swp_offset(pte);
> >         const pte_t *end_ptep = start_ptep + max_nr;
> >         pte_t *ptep = start_ptep + 1;
> >
> >         VM_WARN_ON(max_nr < 1);
> >         VM_WARN_ON(!is_swap_pte(pte));
> >         VM_WARN_ON(non_swap_entry(pte_to_swp_entry(pte)));
> >
> >         while (ptep < end_ptep) {
> >                 pte = ptep_get(ptep);
> >
> >                 if (!pte_same(pte, expected_pte))
> >                         break;
> >
> >                 expected_pte = pte_next_swp_offset(expected_pte);
> >                 ptep++;
> >         }
> >
> >         return ptep - start_ptep;
> > }
>
> Yes.  You are right.
>
> But we may check whether the pte of page0 is same as "vmf->orig_pte -
> folio_page_idx()" (fake code).

right, that is why we are reading and checking PTE0 before calling
swap_pte_batch()
right now.

  folio_ptep = vmf->pte - idx;
  folio_pte = ptep_get(folio_ptep);
  if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
      swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
   goto check_pte;

So, if I understand correctly, you're proposing that we should directly check
PTE0 in swap_pte_batch(). Personally, I don't have any objections to this idea.
However, I'd also like to hear the feedback from Ryan and David :-)

>
> You need to check the pte of page 0 anyway.
>
> >>
> >> > +             if (!is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte)) ||
> >> > +                 swap_pte_batch(folio_ptep, nr, folio_pte, &any_swap_shared) != nr)
> >> > +                     goto check_pte;
> >> > +
> >> > +             start_address = folio_start;
> >> > +             start_pte = folio_ptep;
> >> > +             nr_pages = nr;
> >> > +             entry = folio->swap;
> >> > +             page = &folio->page;
> >> > +     }
> >> > +
> >> > +check_pte:
> >> >       if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
> >> >               goto out_nomap;
> >> >
> >> > @@ -4190,6 +4223,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >> >                        */
> >> >                       exclusive = false;
> >> >               }
> >> > +
> >> > +             /* Reuse the whole large folio iff all entries are exclusive */
> >> > +             if (nr_pages > 1 && any_swap_shared)
> >> > +                     exclusive = false;
> >> >       }
> >> >
> >> >       /*
> >> > @@ -4204,12 +4241,14 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >> >        * We're already holding a reference on the page but haven't mapped it
> >> >        * yet.
> >> >        */
> >> > -     swap_free(entry);
> >> > +     swap_free_nr(entry, nr_pages);
> >> >       if (should_try_to_free_swap(folio, vma, vmf->flags))
> >> >               folio_free_swap(folio);
> >> >
> >> > -     inc_mm_counter(vma->vm_mm, MM_ANONPAGES);
> >> > -     dec_mm_counter(vma->vm_mm, MM_SWAPENTS);
> >> > +     folio_ref_add(folio, nr_pages - 1);
> >> > +     add_mm_counter(vma->vm_mm, MM_ANONPAGES, nr_pages);
> >> > +     add_mm_counter(vma->vm_mm, MM_SWAPENTS, -nr_pages);
> >> > +
> >> >       pte = mk_pte(page, vma->vm_page_prot);
> >> >
> >> >       /*
> >> > @@ -4219,33 +4258,34 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >> >        * exclusivity.
> >> >        */
> >> >       if (!folio_test_ksm(folio) &&
> >> > -         (exclusive || folio_ref_count(folio) == 1)) {
> >> > +         (exclusive || (folio_ref_count(folio) == nr_pages &&
> >> > +                        folio_nr_pages(folio) == nr_pages))) {
> >> >               if (vmf->flags & FAULT_FLAG_WRITE) {
> >> >                       pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> >> >                       vmf->flags &= ~FAULT_FLAG_WRITE;
> >> >               }
> >> >               rmap_flags |= RMAP_EXCLUSIVE;
> >> >       }
> >> > -     flush_icache_page(vma, page);
> >> > +     flush_icache_pages(vma, page, nr_pages);
> >> >       if (pte_swp_soft_dirty(vmf->orig_pte))
> >> >               pte = pte_mksoft_dirty(pte);
> >> >       if (pte_swp_uffd_wp(vmf->orig_pte))
> >> >               pte = pte_mkuffd_wp(pte);
> >> > -     vmf->orig_pte = pte;
> >> >
> >> >       /* ksm created a completely new copy */
> >> >       if (unlikely(folio != swapcache && swapcache)) {
> >> > -             folio_add_new_anon_rmap(folio, vma, vmf->address);
> >> > +             folio_add_new_anon_rmap(folio, vma, start_address);
> >> >               folio_add_lru_vma(folio, vma);
> >> >       } else {
> >> > -             folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> >> > -                                     rmap_flags);
> >> > +             folio_add_anon_rmap_ptes(folio, page, nr_pages, vma, start_address,
> >> > +                                      rmap_flags);
> >> >       }
> >> >
> >> >       VM_BUG_ON(!folio_test_anon(folio) ||
> >> >                       (pte_write(pte) && !PageAnonExclusive(page)));
> >> > -     set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
> >> > -     arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
> >> > +     set_ptes(vma->vm_mm, start_address, start_pte, pte, nr_pages);
> >> > +     vmf->orig_pte = ptep_get(vmf->pte);
> >> > +     arch_do_swap_page(vma->vm_mm, vma, start_address, pte, pte);
> >>
> >> Do we need to call arch_do_swap_page() for each subpage?  IIUC, the
> >> corresponding arch_unmap_one() will be called for each subpage.
> >
> > i actually thought about this very carefully, right now, the only one who
> > needs this is sparc and it doesn't support THP_SWAPOUT at all. and
> > there is no proof doing restoration one by one won't really break sparc.
> > so i'd like to defer this to when sparc really needs THP_SWAPOUT.
>
> Let's ask SPARC developer (Cced) for this.
>
> IMHO, even if we cannot get help, we need to change code with our
> understanding instead of deferring it.

ok. Thanks for Ccing sparc developers.

>
> > on the other hand, it seems really bad we have both
> > arch_swap_restore  - for this, arm64 has moved to using folio
> > and
> > arch_do_swap_page
> >
> > we should somehow unify them later if sparc wants THP_SWPOUT.
> >
> >>
> >> >       folio_unlock(folio);
> >> >       if (folio != swapcache && swapcache) {
> >> > @@ -4269,7 +4309,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >> >       }
> >> >
> >> >       /* No need to invalidate - it was non-present before */
> >> > -     update_mmu_cache_range(vmf, vma, vmf->address, vmf->pte, 1);
> >> > +     update_mmu_cache_range(vmf, vma, start_address, start_pte, nr_pages);
> >> >  unlock:
> >> >       if (vmf->pte)
> >> >               pte_unmap_unlock(vmf->pte, vmf->ptl);
>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry