Re: [RFC PATCH 2/2] mm/khugepaged: Remove compound_pagelist
From: Yang Shi
Date: Tue Sep 26 2023 - 18:13:51 EST
On Fri, Sep 22, 2023 at 9:33 PM Vishal Moola (Oracle)
<vishal.moola@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> 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 >= pte) {
> pte_t pteval = ptep_get(_pte);
> @@ -514,12 +513,7 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> continue;
> folio = 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 -= 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_pagelist)
> + struct collapse_control *cc)
> {
> struct folio *folio = NULL;
> pte_t *_pte;
> @@ -588,19 +581,6 @@ static int __collapse_huge_page_isolate(struct vm_area_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 == 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_area_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 there is
> * enough young pte to justify collapsing the page
> @@ -660,6 +637,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> if (pte_write(pteval))
> writable = true;
>
> + if (folio_test_large(folio)) {
> + _pte += folio_nr_pages(folio) - 1;
> + address += folio_size(folio) - PAGE_SIZE;
> + }
> }
>
> if (unlikely(!writable)) {
> @@ -673,7 +654,7 @@ static int __collapse_huge_page_isolate(struct vm_area_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_area_struct *vma,
> static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> struct vm_area_struct *vma,
> unsigned long address,
> - spinlock_t *ptl,
> - struct list_head *compound_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 = 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(src_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_pagelist)
> + struct vm_area_struct *vma)
> {
> spinlock_t *pmd_ptl;
>
> @@ -753,7 +720,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> * 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 *pte,
> * @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 == SCAN_SUCCEED))
> - __collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
> - compound_pagelist);
> + __collapse_huge_page_copy_succeeded(pte, vma, address, ptl);
> 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 = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> if (pte) {
> - result = __collapse_huge_page_isolate(vma, address, pte, cc,
> - &compound_pagelist);
> + result = __collapse_huge_page_isolate(vma, address, pte, cc);
> spin_unlock(pte_ptl);
> } else {
> result = 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 = __collapse_huge_page_copy(pte, hpage, pmd, _pmd,
> - vma, address, pte_ptl,
> - &compound_pagelist);
> + vma, address, pte_ptl);
> pte_unmap(pte);
> if (unlikely(result != SCAN_SUCCEED))
> goto out_up_write;
> --
> 2.40.1
>