Re: [PATCH RFC 4/6] mm: support large folios swapin as a whole

[Chris Li]

On Thu, Jan 18, 2024 at 3:12 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>
> On an embedded system like Android, more than half of anon memory is actually
> in swap devices such as zRAM. For example, while an app is switched to back-
> ground, its most memory might be swapped-out.
>
> Now we have mTHP features, unfortunately, if we don't support large folios
> swap-in, once those large folios are swapped-out, we immediately lose the
> performance gain we can get through large folios and hardware optimization
> such as CONT-PTE.
>
> This patch brings up mTHP swap-in support. Right now, we limit mTHP swap-in
> to those contiguous swaps which were likely swapped out from mTHP as a whole.
>
> On the other hand, the current implementation only covers the SWAP_SYCHRONOUS
> case. It doesn't support swapin_readahead as large folios yet.
>
> Right now, we are re-faulting large folios which are still in swapcache as a
> whole, this can effectively decrease extra loops and early-exitings which we
> have increased in arch_swap_restore() while supporting MTE restore for folios
> rather than page.
>
> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> ---
>  mm/memory.c | 108 +++++++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 94 insertions(+), 14 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index f61a48929ba7..928b3f542932 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -107,6 +107,8 @@ EXPORT_SYMBOL(mem_map);
>  static vm_fault_t do_fault(struct vm_fault *vmf);
>  static vm_fault_t do_anonymous_page(struct vm_fault *vmf);
>  static bool vmf_pte_changed(struct vm_fault *vmf);
> +static struct folio *alloc_anon_folio(struct vm_fault *vmf,
> +                                     bool (*pte_range_check)(pte_t *, int));
>
>  /*
>   * Return true if the original pte was a uffd-wp pte marker (so the pte was
> @@ -3784,6 +3786,34 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
>         return VM_FAULT_SIGBUS;
>  }
>
> +static bool pte_range_swap(pte_t *pte, int nr_pages)
> +{
> +       int i;
> +       swp_entry_t entry;
> +       unsigned type;
> +       pgoff_t start_offset;
> +
> +       entry = pte_to_swp_entry(ptep_get_lockless(pte));
> +       if (non_swap_entry(entry))
> +               return false;
> +       start_offset = swp_offset(entry);
> +       if (start_offset % nr_pages)
> +               return false;
> +
> +       type = swp_type(entry);
> +       for (i = 1; i < nr_pages; i++) {
> +               entry = pte_to_swp_entry(ptep_get_lockless(pte + i));
> +               if (non_swap_entry(entry))
> +                       return false;
> +               if (swp_offset(entry) != start_offset + i)
> +                       return false;
> +               if (swp_type(entry) != type)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
>  /*
>   * We enter with non-exclusive mmap_lock (to exclude vma changes,
>   * but allow concurrent faults), and pte mapped but not yet locked.
> @@ -3804,6 +3834,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         pte_t pte;
>         vm_fault_t ret = 0;
>         void *shadow = NULL;
> +       int nr_pages = 1;
> +       unsigned long start_address;
> +       pte_t *start_pte;
>
>         if (!pte_unmap_same(vmf))
>                 goto out;
> @@ -3868,13 +3901,20 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                 if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
>                     __swap_count(entry) == 1) {
>                         /* skip swapcache */
> -                       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0,
> -                                               vma, vmf->address, false);
> +                       folio = alloc_anon_folio(vmf, pte_range_swap);
>                         page = &folio->page;
>                         if (folio) {
>                                 __folio_set_locked(folio);
>                                 __folio_set_swapbacked(folio);
>
> +                               if (folio_test_large(folio)) {
> +                                       unsigned long start_offset;
> +
> +                                       nr_pages = folio_nr_pages(folio);
> +                                       start_offset = swp_offset(entry) & ~(nr_pages - 1);
> +                                       entry = swp_entry(swp_type(entry), start_offset);
> +                               }
> +
>                                 if (mem_cgroup_swapin_charge_folio(folio,
>                                                         vma->vm_mm, GFP_KERNEL,
>                                                         entry)) {
> @@ -3980,6 +4020,39 @@ 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);
> +
> +       start_address = vmf->address;
> +       start_pte = vmf->pte;
> +       if (folio_test_large(folio)) {
> +               unsigned long nr = folio_nr_pages(folio);
> +               unsigned long addr = ALIGN_DOWN(vmf->address, nr * PAGE_SIZE);
> +               pte_t *pte_t = vmf->pte - (vmf->address - addr) / PAGE_SIZE;

I forgot about one comment here.
Please change the variable name other than "pte_t", it is a bit
strange to use the typedef name as variable name here.

Chris

> +
> +               /*
> +                * case 1: we are allocating large_folio, try to map it as a whole
> +                * iff the swap entries are still entirely mapped;
> +                * case 2: we hit a large folio in swapcache, and all swap entries
> +                * are still entirely mapped, try to map a large folio as a whole.
> +                * otherwise, map only the faulting page within the large folio
> +                * which is swapcache
> +                */
> +               if (pte_range_swap(pte_t, nr)) {
> +                       start_address = addr;
> +                       start_pte = pte_t;
> +                       if (unlikely(folio == swapcache)) {
> +                               /*
> +                                * the below has been done before swap_read_folio()
> +                                * for case 1
> +                                */
> +                               nr_pages = nr;
> +                               entry = pte_to_swp_entry(ptep_get(start_pte));
> +                               page = &folio->page;
> +                       }
> +               } else if (nr_pages > 1) { /* ptes have changed for case 1 */
> +                       goto out_nomap;
> +               }
> +       }
> +
>         if (unlikely(!vmf->pte || !pte_same(ptep_get(vmf->pte), vmf->orig_pte)))
>                 goto out_nomap;
>
> @@ -4047,12 +4120,14 @@ 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_nr_free(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);
>
>         /*
> @@ -4062,14 +4137,14 @@ 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)) {
>                 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))
> @@ -4081,14 +4156,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>                 folio_add_new_anon_rmap(folio, vma, vmf->address);
>                 folio_add_lru_vma(folio, vma);
>         } else {
> -               folio_add_anon_rmap_pte(folio, page, vma, vmf->address,
> +               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);
> +
> +       arch_do_swap_page(vma->vm_mm, vma, start_address, pte, vmf->orig_pte);
>
>         folio_unlock(folio);
>         if (folio != swapcache && swapcache) {
> @@ -4105,6 +4181,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>         }
>
>         if (vmf->flags & FAULT_FLAG_WRITE) {
> +               if (folio_test_large(folio) && nr_pages > 1)
> +                       vmf->orig_pte = ptep_get(vmf->pte);
> +
>                 ret |= do_wp_page(vmf);
>                 if (ret & VM_FAULT_ERROR)
>                         ret &= VM_FAULT_ERROR;
> @@ -4112,7 +4191,7 @@ 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);
> @@ -4148,7 +4227,8 @@ static bool pte_range_none(pte_t *pte, int nr_pages)
>         return true;
>  }
>
> -static struct folio *alloc_anon_folio(struct vm_fault *vmf)
> +static struct folio *alloc_anon_folio(struct vm_fault *vmf,
> +                                     bool (*pte_range_check)(pte_t *, int))
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         struct vm_area_struct *vma = vmf->vma;
> @@ -4190,7 +4270,7 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>         order = highest_order(orders);
>         while (orders) {
>                 addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> -               if (pte_range_none(pte + pte_index(addr), 1 << order))
> +               if (pte_range_check(pte + pte_index(addr), 1 << order))
>                         break;
>                 order = next_order(&orders, order);
>         }
> @@ -4269,7 +4349,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>         if (unlikely(anon_vma_prepare(vma)))
>                 goto oom;
>         /* Returns NULL on OOM or ERR_PTR(-EAGAIN) if we must retry the fault */
> -       folio = alloc_anon_folio(vmf);
> +       folio = alloc_anon_folio(vmf, pte_range_none);
>         if (IS_ERR(folio))
>                 return 0;
>         if (!folio)
> --
> 2.34.1
>
>