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

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Date:   Thu, 20 Apr 2017 08:43:12 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        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

Hi, Johannes,

Johannes Weiner <hannes@...xchg.org> writes:

> 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.

I just found there are unlikely() encloses PageTransHuge() in the
original add_to_swap(), so I just follow the original style.  But I
don't think they make much sense too.  Will remove them in the next
version.

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

Sorry and will do that in the future.

Best Regards,
Huang, Ying