Re: [PATCH -mm -v9 1/3] mm, THP, swap: Delay splitting THP during swap out

[Johannes Weiner]

On Wed, Apr 19, 2017 at 03:06:23PM +0800, Huang, Ying wrote:
> @@ -206,17 +212,34 @@ int add_to_swap(struct page *page, struct list_head *list)
>  	 */
>  	err = add_to_swap_cache(page, entry,
>  			__GFP_HIGH|__GFP_NOMEMALLOC|__GFP_NOWARN);
> -
> -	if (!err) {
> -		return 1;
> -	} else {	/* -ENOMEM radix-tree allocation failure */
> +	/* -ENOMEM radix-tree allocation failure */
> +	if (err)
>  		/*
>  		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
>  		 * clear SWAP_HAS_CACHE flag.
>  		 */
> -		swapcache_free(entry);
> -		return 0;
> +		goto fail_free;
> +
> +	if (unlikely(PageTransHuge(page))) {
> +		err = split_huge_page_to_list(page, list);
> +		if (err) {
> +			delete_from_swap_cache(page);
> +			return 0;
> +		}
>  	}
> +
> +	return 1;
> +
> +fail_free:
> +	if (unlikely(PageTransHuge(page)))
> +		swapcache_free_cluster(entry);
> +	else
> +		swapcache_free(entry);
> +fail:
> +	if (unlikely(PageTransHuge(page)) &&
> +	    !split_huge_page_to_list(page, list))
> +		goto retry;

May I ask why you added the unlikelies there? Can you generally say
THPs are unlikely in this path? Is the swap-out path so hot that
branch layout is critical? I doubt either is true.

Also please mention changes like these in the changelog next time.