Re: [PATCH] mm, thp: Do not queue fully unmapped pages for deferred split

[Michal Hocko]

On Fri 13-09-19 12:18:49, Kirill A. Shutemov wrote:
> Adding fully unmapped pages into deferred split queue is not productive:
> these pages are about to be freed or they are pinned and cannot be split
> anyway.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  mm/rmap.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 003377e24232..45388f1bf317 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1271,12 +1271,20 @@ static void page_remove_anon_compound_rmap(struct page *page)
>  	if (TestClearPageDoubleMap(page)) {
>  		/*
>  		 * Subpages can be mapped with PTEs too. Check how many of
> -		 * themi are still mapped.
> +		 * them are still mapped.
>  		 */
>  		for (i = 0, nr = 0; i < HPAGE_PMD_NR; i++) {
>  			if (atomic_add_negative(-1, &page[i]._mapcount))
>  				nr++;
>  		}
> +
> +		/*
> +		 * Queue the page for deferred split if at least one small
> +		 * page of the compound page is unmapped, but at least one
> +		 * small page is still mapped.
> +		 */
> +		if (nr && nr < HPAGE_PMD_NR)
> +			deferred_split_huge_page(page);

You've set nr to zero in the for loop so this cannot work AFAICS.

>  	} else {
>  		nr = HPAGE_PMD_NR;
>  	}
> @@ -1284,10 +1292,8 @@ static void page_remove_anon_compound_rmap(struct page *page)
>  	if (unlikely(PageMlocked(page)))
>  		clear_page_mlock(page);
>  
> -	if (nr) {
> +	if (nr)
>  		__mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, -nr);
> -		deferred_split_huge_page(page);
> -	}
>  }
>  
>  /**
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs