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

Johannes Weiner
2017-04-19T16:00:38Z

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.
*/