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

["Huang, Ying" <ying.huang@intel.com>]

Barry Song <21cnbao@gmail.com> writes:

> On Tue, Apr 16, 2024 at 2:27 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>>
>> Added Khalid for arch_do_swap_page().
>>
>> Barry Song <21cnbao@gmail.com> writes:
>>
>> > On Mon, Apr 15, 2024 at 8:39 PM Huang, Ying <ying.huang@intel.com> wrote:
>> >>
>> >> Barry Song <21cnbao@gmail.com> 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.

>>
>> 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