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

[Yang Shi <shy828301@gmail.com>]

On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
<kirill@shutemov.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@linux.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));

We need count in the number of base pages. The same counter is
modified by vmscan in base page unit. Also need modify the inc path.

>         unlock_page(page);
>         putback_lru_page(page);
> @@ -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

s/compount/compound

> +                        * already
> +                        */
> +                       list_for_each_entry(p, &compound_pagelist, lru) {
> +                               if (page ==  p)
> +                                       break;
> +                       }
> +                       if (page ==  p)
> +                               continue;

I don't quite understand why we need the above check. My understanding
is when we scan the ptes, once the first PTE mapped subpage is found,
then the THP would be added into compound_pagelist, then the later
loop would find the same THP on the list then just break the loop. Did
I miss anything?

> +               }
>
>                 /*
>                  * 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
>
>