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

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Date:   Wed, 19 Apr 2017 11:52:52 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Ebru Akagunduz <ebru.akagunduz@...il.com>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...nel.org>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, cgroups@...r.kernel.org
Subject: Re: [PATCH -mm -v9 1/3] mm, THP, swap: Delay splitting THP during
 swap out

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.