Re: [PATCH -mm -v10 3/3] mm, THP, swap: Enable THP swap optimization only if has compound map

[Johannes Weiner]

On Tue, Apr 25, 2017 at 08:56:58PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> If there is no compound map for a THP (Transparent Huge Page), it is
> possible that the map count of some sub-pages of the THP is 0.  So it
> is better to split the THP before swapping out. In this way, the
> sub-pages not mapped will be freed, and we can avoid the unnecessary
> swap out operations for these sub-pages.
> 
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

CC Kirill to double check the reasoning here

> ---
>  mm/swap_state.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 006d91d8fc53..13f83c6bb1b4 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -192,9 +192,19 @@ int add_to_swap(struct page *page, struct list_head *list)
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	VM_BUG_ON_PAGE(!PageUptodate(page), page);
>  
> -	/* cannot split, skip it */
> -	if (PageTransHuge(page) && !can_split_huge_page(page, NULL))
> -		return 0;
> +	if (PageTransHuge(page)) {
> +		/* cannot split, skip it */
> +		if (!can_split_huge_page(page, NULL))
> +			return 0;
> +		/*
> +		 * Split pages without a PMD map right away. Chances
> +		 * are some or all of the tail pages can be freed
> +		 * without IO.
> +		 */
> +		if (!compound_mapcount(page) &&
> +		    split_huge_page_to_list(page, list))
> +			return 0;
> +	}
>  
>  retry:
>  	entry = get_swap_page(page);
> -- 
> 2.11.0