Re: [PATCH 3/7] khugepaged: Drain LRU add pagevec to get rid of extra pins

[Yang Shi]

On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
>
> __collapse_huge_page_isolate() may fail due to extra pin in the LRU add
> pagevec. It's petty common for swapin case: we swap in pages just to
> fail due to the extra pin.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/khugepaged.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 14d7afc90786..39e0994abeb8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -585,11 +585,19 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>                  * The page must only be referenced by the scanned process
>                  * and page swap cache.
>                  */
> +               if (page_count(page) != 1 + PageSwapCache(page)) {
> +                       /*
> +                        * Drain pagevec and retry just in case we can get rid
> +                        * of the extra pin, like in swapin case.
> +                        */
> +                       lru_add_drain();

This is definitely correct.

I'm wondering if we need one more lru_add_drain() before PageLRU check
in khugepaged_scan_pmd() or not? The page might be in lru cache then
get skipped. This would improve the success rate.

Reviewed-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>

> +               }
>                 if (page_count(page) != 1 + PageSwapCache(page)) {
>                         unlock_page(page);
>                         result = SCAN_PAGE_COUNT;
>                         goto out;
>                 }
> +
>                 if (pte_write(pteval)) {
>                         writable = true;
>                 } else {
> --
> 2.26.0
>
>