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References: <20230922193639.10158-1-vishal.moola@gmail.com> <20230922193639.10158-3-vishal.moola@gmail.com>
In-Reply-To: <20230922193639.10158-3-vishal.moola@gmail.com>
From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 26 Sep 2023 15:07:18 -0700
Message-ID: <CAHbLzkqt8LFFDH2a7+xaQmncwiK=Ynv6vHs9d=TDNjp9_wJ4rw@mail.gmail.com>
Subject: Re: [RFC PATCH 2/2] mm/khugepaged: Remove compound_pagelist
To:     "Vishal Moola (Oracle)" <vishal.moola@gmail.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org
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On Fri, Sep 22, 2023 at 9:33=E2=80=AFPM Vishal Moola (Oracle)
<vishal.moola@gmail.com> wrote:
>
> Currently, khugepaged builds a compound_pagelist while scanning, which
> is used to properly account for compound pages. We can now account
> for a compound page as a singular folio instead, so remove this list.
>
> Large folios are guaranteed to have consecutive ptes and addresses, so
> once the first pte of a large folio is found skip over the rest.

The address space may just map a partial folio, for example, in the
extreme case the HUGE_PMD size range may have HUGE_PMD_NR folios with
mapping one subpage from each folio per PTE. So assuming the PTE
mapped folio is mapped consecutively may be wrong.

Please refer to collapse_compound_extreme() in
tools/testing/selftests/mm/khugepaged.c.

>
> This helps convert khugepaged to use folios. It removes 3 compound_head
> calls in __collapse_huge_page_copy_succeeded(), and removes 980 bytes of
> kernel text.
>
> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> ---
>  mm/khugepaged.c | 76 ++++++++++++-------------------------------------
>  1 file changed, 18 insertions(+), 58 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index f46a7a7c489f..b6c7d55a8231 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -498,10 +498,9 @@ static void release_pte_page(struct page *page)
>         release_pte_folio(page_folio(page));
>  }
>
> -static void release_pte_pages(pte_t *pte, pte_t *_pte,
> -               struct list_head *compound_pagelist)
> +static void release_pte_folios(pte_t *pte, pte_t *_pte)
>  {
> -       struct folio *folio, *tmp;
> +       struct folio *folio;
>
>         while (--_pte >=3D pte) {
>                 pte_t pteval =3D ptep_get(_pte);
> @@ -514,12 +513,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pt=
e,
>                         continue;
>                 folio =3D pfn_folio(pfn);
>                 if (folio_test_large(folio))
> -                       continue;
> -               release_pte_folio(folio);
> -       }
> -
> -       list_for_each_entry_safe(folio, tmp, compound_pagelist, lru) {
> -               list_del(&folio->lru);
> +                       _pte -=3D folio_nr_pages(folio) - 1;
>                 release_pte_folio(folio);
>         }
>  }
> @@ -538,8 +532,7 @@ static bool is_refcount_suitable(struct page *page)
>  static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                                         unsigned long address,
>                                         pte_t *pte,
> -                                       struct collapse_control *cc,
> -                                       struct list_head *compound_pageli=
st)
> +                                       struct collapse_control *cc)
>  {
>         struct folio *folio =3D NULL;
>         pte_t *_pte;
> @@ -588,19 +581,6 @@ static int __collapse_huge_page_isolate(struct vm_ar=
ea_struct *vma,
>                         }
>                 }
>
> -               if (folio_test_large(folio)) {
> -                       struct folio *f;
> -
> -                       /*
> -                        * Check if we have dealt with the compound page
> -                        * already
> -                        */
> -                       list_for_each_entry(f, compound_pagelist, lru) {
> -                               if (folio =3D=3D f)
> -                                       goto next;
> -                       }
> -               }
> -
>                 /*
>                  * We can do it before isolate_lru_page because the
>                  * page can't be freed from under us. NOTE: PG_lock
> @@ -644,9 +624,6 @@ static int __collapse_huge_page_isolate(struct vm_are=
a_struct *vma,
>                 VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>                 VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
>
> -               if (folio_test_large(folio))
> -                       list_add_tail(&folio->lru, compound_pagelist);
> -next:
>                 /*
>                  * If collapse was initiated by khugepaged, check that th=
ere is
>                  * enough young pte to justify collapsing the page
> @@ -660,6 +637,10 @@ static int __collapse_huge_page_isolate(struct vm_ar=
ea_struct *vma,
>                 if (pte_write(pteval))
>                         writable =3D true;
>
> +               if (folio_test_large(folio)) {
> +                       _pte +=3D folio_nr_pages(folio) - 1;
> +                       address +=3D folio_size(folio) - PAGE_SIZE;
> +               }
>         }
>
>         if (unlikely(!writable)) {
> @@ -673,7 +654,7 @@ static int __collapse_huge_page_isolate(struct vm_are=
a_struct *vma,
>                 return result;
>         }
>  out:
> -       release_pte_pages(pte, _pte, compound_pagelist);
> +       release_pte_folios(pte, _pte);
>         trace_mm_collapse_huge_page_isolate(&folio->page, none_or_zero,
>                                             referenced, writable, result)=
;
>         return result;
> @@ -682,11 +663,9 @@ static int __collapse_huge_page_isolate(struct vm_ar=
ea_struct *vma,
>  static void __collapse_huge_page_copy_succeeded(pte_t *pte,
>                                                 struct vm_area_struct *vm=
a,
>                                                 unsigned long address,
> -                                               spinlock_t *ptl,
> -                                               struct list_head *compoun=
d_pagelist)
> +                                               spinlock_t *ptl)
>  {
>         struct page *src_page;
> -       struct page *tmp;
>         pte_t *_pte;
>         pte_t pteval;
>
> @@ -706,8 +685,7 @@ static void __collapse_huge_page_copy_succeeded(pte_t=
 *pte,
>                         }
>                 } else {
>                         src_page =3D pte_page(pteval);
> -                       if (!PageCompound(src_page))
> -                               release_pte_page(src_page);
> +                       release_pte_page(src_page);
>                         /*
>                          * ptl mostly unnecessary, but preempt has to
>                          * be disabled to update the per-cpu stats
> @@ -720,23 +698,12 @@ static void __collapse_huge_page_copy_succeeded(pte=
_t *pte,
>                         free_page_and_swap_cache(src_page);
>                 }
>         }
> -
> -       list_for_each_entry_safe(src_page, tmp, compound_pagelist, lru) {
> -               list_del(&src_page->lru);
> -               mod_node_page_state(page_pgdat(src_page),
> -                                   NR_ISOLATED_ANON + page_is_file_lru(s=
rc_page),
> -                                   -compound_nr(src_page));
> -               unlock_page(src_page);
> -               free_swap_cache(src_page);
> -               putback_lru_page(src_page);
> -       }
>  }
>
>  static void __collapse_huge_page_copy_failed(pte_t *pte,
>                                              pmd_t *pmd,
>                                              pmd_t orig_pmd,
> -                                            struct vm_area_struct *vma,
> -                                            struct list_head *compound_p=
agelist)
> +                                            struct vm_area_struct *vma)
>  {
>         spinlock_t *pmd_ptl;
>
> @@ -753,7 +720,7 @@ static void __collapse_huge_page_copy_failed(pte_t *p=
te,
>          * Release both raw and compound pages isolated
>          * in __collapse_huge_page_isolate.
>          */
> -       release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> +       release_pte_folios(pte, pte + HPAGE_PMD_NR);
>  }
>
>  /*
> @@ -769,7 +736,6 @@ static void __collapse_huge_page_copy_failed(pte_t *p=
te,
>   * @vma: the original raw pages' virtual memory area
>   * @address: starting address to copy
>   * @ptl: lock on raw pages' PTEs
> - * @compound_pagelist: list that stores compound pages
>   */
>  static int __collapse_huge_page_copy(pte_t *pte,
>                                      struct page *page,
> @@ -777,8 +743,7 @@ static int __collapse_huge_page_copy(pte_t *pte,
>                                      pmd_t orig_pmd,
>                                      struct vm_area_struct *vma,
>                                      unsigned long address,
> -                                    spinlock_t *ptl,
> -                                    struct list_head *compound_pagelist)
> +                                    spinlock_t *ptl)
>  {
>         struct page *src_page;
>         pte_t *_pte;
> @@ -804,11 +769,9 @@ static int __collapse_huge_page_copy(pte_t *pte,
>         }
>
>         if (likely(result =3D=3D SCAN_SUCCEED))
> -               __collapse_huge_page_copy_succeeded(pte, vma, address, pt=
l,
> -                                                   compound_pagelist);
> +               __collapse_huge_page_copy_succeeded(pte, vma, address, pt=
l);
>         else
> -               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> -                                                compound_pagelist);
> +               __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma)=
;
>
>         return result;
>  }
> @@ -1081,7 +1044,6 @@ static int collapse_huge_page(struct mm_struct *mm,=
 unsigned long address,
>                               int referenced, int unmapped,
>                               struct collapse_control *cc)
>  {
> -       LIST_HEAD(compound_pagelist);
>         pmd_t *pmd, _pmd;
>         pte_t *pte;
>         pgtable_t pgtable;
> @@ -1168,8 +1130,7 @@ static int collapse_huge_page(struct mm_struct *mm,=
 unsigned long address,
>
>         pte =3D pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
>         if (pte) {
> -               result =3D __collapse_huge_page_isolate(vma, address, pte=
, cc,
> -                                                     &compound_pagelist)=
;
> +               result =3D __collapse_huge_page_isolate(vma, address, pte=
, cc);
>                 spin_unlock(pte_ptl);
>         } else {
>                 result =3D SCAN_PMD_NULL;
> @@ -1198,8 +1159,7 @@ static int collapse_huge_page(struct mm_struct *mm,=
 unsigned long address,
>         anon_vma_unlock_write(vma->anon_vma);
>
>         result =3D __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> -                                          vma, address, pte_ptl,
> -                                          &compound_pagelist);
> +                                          vma, address, pte_ptl);
>         pte_unmap(pte);
>         if (unlikely(result !=3D SCAN_SUCCEED))
>                 goto out_up_write;
> --
> 2.40.1
>