Re: [PATCH -mm -v9 3/3] mm, THP, swap: Enable THP swap optimization only if has compound map

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Apr 19, 2017 at 03:06:25PM +0800, Huang, Ying wrote:
> From: Huang Ying <ying.huang@intel.com>
> 
> 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@cmpxchg.org>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> ---
>  mm/swap_state.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 3a3217f68937..b025c9878e5e 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -192,9 +192,15 @@ 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 (unlikely(PageTransHuge(page)) && !can_split_huge_page(page, NULL))
> -		return 0;
> +	if (unlikely(PageTransHuge(page))) {
> +		/* cannot split, skip it */
> +		if (!can_split_huge_page(page, NULL))
> +			return 0;
> +		/* fallback to split huge page firstly if no PMD map */
> +		if (!compound_mapcount(page) &&
> +		    split_huge_page_to_list(page, list))
> +			return 0;
> +	}

This looks good to me, but could you please elaborate the comment a
little bit with what you have in the changelog? Something like:

	/*
	 * Split pages without a PMD map right away. Chances are
	 * some or all of the tail pages can be freed without IO.
	 */

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