Re: [PATCH v3 5/5] mm/khugepaged: Convert collapse_pte_mapped_thp() to use folios

[Yang Shi <shy828301@gmail.com>]

On Fri, Oct 20, 2023 at 11:34 AM Vishal Moola (Oracle)
<vishal.moola@gmail.com> wrote:
>
> This removes 2 calls to compound_head() and helps convert khugepaged to
> use folios throughout.
>
> Previously, if the address passed to collapse_pte_mapped_thp()
> corresponded to a tail page, the scan would fail immediately. Using
> filemap_lock_folio() we get the corresponding folio back and try to
> operate on the folio instead.
>
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/khugepaged.c | 45 ++++++++++++++++++++-------------------------
>  1 file changed, 20 insertions(+), 25 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 6a7184cd291b..bc2d8ff269c7 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1477,7 +1477,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>         bool notified = false;
>         unsigned long haddr = addr & HPAGE_PMD_MASK;
>         struct vm_area_struct *vma = vma_lookup(mm, haddr);
> -       struct page *hpage;
> +       struct folio *folio;
>         pte_t *start_pte, *pte;
>         pmd_t *pmd, pgt_pmd;
>         spinlock_t *pml = NULL, *ptl;
> @@ -1510,19 +1510,14 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>         if (userfaultfd_wp(vma))
>                 return SCAN_PTE_UFFD_WP;
>
> -       hpage = find_lock_page(vma->vm_file->f_mapping,
> +       folio = filemap_lock_folio(vma->vm_file->f_mapping,
>                                linear_page_index(vma, haddr));
> -       if (!hpage)
> +       if (IS_ERR(folio))
>                 return SCAN_PAGE_NULL;
>
> -       if (!PageHead(hpage)) {
> -               result = SCAN_FAIL;
> -               goto drop_hpage;
> -       }
> -
> -       if (compound_order(hpage) != HPAGE_PMD_ORDER) {
> +       if (folio_order(folio) != HPAGE_PMD_ORDER) {
>                 result = SCAN_PAGE_COMPOUND;
> -               goto drop_hpage;
> +               goto drop_folio;
>         }
>
>         result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
> @@ -1536,13 +1531,13 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>                  */
>                 goto maybe_install_pmd;
>         default:
> -               goto drop_hpage;
> +               goto drop_folio;
>         }
>
>         result = SCAN_FAIL;
>         start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
>         if (!start_pte)         /* mmap_lock + page lock should prevent this */
> -               goto drop_hpage;
> +               goto drop_folio;
>
>         /* step 1: check all mapped PTEs are to the right huge page */
>         for (i = 0, addr = haddr, pte = start_pte;
> @@ -1567,7 +1562,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>                  * Note that uprobe, debugger, or MAP_PRIVATE may change the
>                  * page table, but the new page will not be a subpage of hpage.
>                  */
> -               if (hpage + i != page)
> +               if (folio_page(folio, i) != page)
>                         goto abort;
>         }
>
> @@ -1582,7 +1577,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>          * page_table_lock) ptl nests inside pml. The less time we hold pml,
>          * the better; but userfaultfd's mfill_atomic_pte() on a private VMA
>          * inserts a valid as-if-COWed PTE without even looking up page cache.
> -        * So page lock of hpage does not protect from it, so we must not drop
> +        * So page lock of folio does not protect from it, so we must not drop
>          * ptl before pgt_pmd is removed, so uffd private needs pml taken now.
>          */
>         if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED))
> @@ -1606,7 +1601,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>                         continue;
>                 /*
>                  * We dropped ptl after the first scan, to do the mmu_notifier:
> -                * page lock stops more PTEs of the hpage being faulted in, but
> +                * page lock stops more PTEs of the folio being faulted in, but
>                  * does not stop write faults COWing anon copies from existing
>                  * PTEs; and does not stop those being swapped out or migrated.
>                  */
> @@ -1615,7 +1610,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>                         goto abort;
>                 }
>                 page = vm_normal_page(vma, addr, ptent);
> -               if (hpage + i != page)
> +               if (folio_page(folio, i) != page)
>                         goto abort;
>
>                 /*
> @@ -1634,8 +1629,8 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>
>         /* step 3: set proper refcount and mm_counters. */
>         if (nr_ptes) {
> -               page_ref_sub(hpage, nr_ptes);
> -               add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes);
> +               folio_ref_sub(folio, nr_ptes);
> +               add_mm_counter(mm, mm_counter_file(&folio->page), -nr_ptes);
>         }
>
>         /* step 4: remove empty page table */
> @@ -1659,14 +1654,14 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>  maybe_install_pmd:
>         /* step 5: install pmd entry */
>         result = install_pmd
> -                       ? set_huge_pmd(vma, haddr, pmd, hpage)
> +                       ? set_huge_pmd(vma, haddr, pmd, &folio->page)
>                         : SCAN_SUCCEED;
> -       goto drop_hpage;
> +       goto drop_folio;
>  abort:
>         if (nr_ptes) {
>                 flush_tlb_mm(mm);
> -               page_ref_sub(hpage, nr_ptes);
> -               add_mm_counter(mm, mm_counter_file(hpage), -nr_ptes);
> +               folio_ref_sub(folio, nr_ptes);
> +               add_mm_counter(mm, mm_counter_file(&folio->page), -nr_ptes);
>         }
>         if (start_pte)
>                 pte_unmap_unlock(start_pte, ptl);
> @@ -1674,9 +1669,9 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
>                 spin_unlock(pml);
>         if (notified)
>                 mmu_notifier_invalidate_range_end(&range);
> -drop_hpage:
> -       unlock_page(hpage);
> -       put_page(hpage);
> +drop_folio:
> +       folio_unlock(folio);
> +       folio_put(folio);
>         return result;
>  }
>
> --
> 2.40.1
>