linux-kernel - Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

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Date:   Fri, 27 Mar 2020 13:45:55 -0700
From:   Yang Shi <shy828301@...il.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
<kirill@...temov.name> wrote:
>
> We can collapse PTE-mapped compound pages. We only need to avoid
> handling them more than once: lock/unlock page only once if it's present
> in the PMD range multiple times as it handled on compound level. The
> same goes for LRU isolation and putpack.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b47edfe57f7b..c8c2c463095c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
>
>  static void release_pte_page(struct page *page)
>  {
> +       /*
> +        * We need to unlock and put compound page on LRU only once.
> +        * The rest of the pages have to be locked and not on LRU here.
> +        */
> +       VM_BUG_ON_PAGE(!PageCompound(page) &&
> +                       (!PageLocked(page) && PageLRU(page)), page);
> +
> +       if (!PageLocked(page))
> +               return;
> +
> +       page = compound_head(page);
>         dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
>         unlock_page(page);
>         putback_lru_page(page);

BTW, wouldn't this unlock the whole THP and put it back to LRU? Then
we may copy the following PTE mapped pages with page unlocked and on
LRU. I don't see critical problem, just the pages might be on and off
LRU by others, i.e. vmscan, compaction, migration, etc. But no one
could take the page away since try_to_unmap() would fail, but not very
productive.


> @@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>         pte_t *_pte;
>         int none_or_zero = 0, result = 0, referenced = 0;
>         bool writable = false;
> +       LIST_HEAD(compound_pagelist);
>
>         for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
>              _pte++, address += PAGE_SIZE) {
> @@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                         goto out;
>                 }
>
> -               /* TODO: teach khugepaged to collapse THP mapped with pte */
> +               VM_BUG_ON_PAGE(!PageAnon(page), page);
> +
>                 if (PageCompound(page)) {
> -                       result = SCAN_PAGE_COMPOUND;
> -                       goto out;
> -               }
> +                       struct page *p;
> +                       page = compound_head(page);
>
> -               VM_BUG_ON_PAGE(!PageAnon(page), page);
> +                       /*
> +                        * Check if we have dealt with the compount page
> +                        * already
> +                        */
> +                       list_for_each_entry(p, &compound_pagelist, lru) {
> +                               if (page ==  p)
> +                                       break;
> +                       }
> +                       if (page ==  p)
> +                               continue;
> +               }
>
>                 /*
>                  * We can do it before isolate_lru_page because the
> @@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                     page_is_young(page) || PageReferenced(page) ||
>                     mmu_notifier_test_young(vma->vm_mm, address))
>                         referenced++;
> +
> +               if (PageCompound(page))
> +                       list_add_tail(&page->lru, &compound_pagelist);
>         }
>         if (likely(writable)) {
>                 if (likely(referenced)) {
> @@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>                         goto out_unmap;
>                 }
>
> -               /* TODO: teach khugepaged to collapse THP mapped with pte */
> -               if (PageCompound(page)) {
> -                       result = SCAN_PAGE_COMPOUND;
> -                       goto out_unmap;
> -               }
> +               page = compound_head(page);
>
>                 /*
>                  * Record which node the original page is from and save this
> --
> 2.26.0
>
>