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

[Barry Song]

On Tue, Apr 16, 2024 at 2:41 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > 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 :-)
>
> I mean that we can replace
>
>         !is_swap_pte(folio_pte) || non_swap_entry(pte_to_swp_entry(folio_pte))
>
> in above code with pte_same() with constructed expected first pte.

Got it. It could be quite tricky, especially with considerations like
pte_swp_soft_dirty, pte_swp_exclusive, and pte_swp_uffd_wp. We might
require a helper function similar to pte_next_swp_offset() but capable of
moving both forward and backward. For instance:

pte_move_swp_offset(pte_t pte, long delta)

pte_next_swp_offset can insteadly call it by:
pte_move_swp_offset(pte, 1);

Is it what you are proposing?

>
> >>
> >> 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;
> >> >> >       }
> >> >> >
>
> [snip]
>
> --
> Best Regards,
> Huang, Ying