linux-kernel - Re: [PATCH 08/10] mm/hugetlb: return non-isolated page in the loop instead of break and check

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Date:   Fri, 7 Aug 2020 22:32:29 +0800
From:   Wei Yang <richard.weiyang@...ux.alibaba.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/10] mm/hugetlb: return non-isolated page in the loop
 instead of break and check

On Fri, Aug 07, 2020 at 09:09:31PM +0800, Baoquan He wrote:
>On 08/07/20 at 05:12pm, Wei Yang wrote:
>> Function dequeue_huge_page_node_exact() iterates the free list and
>> return the first non-isolated one.
>> 
>> Instead of break and check the loop variant, we could return in the loop
>> directly. This could reduce some redundant check.
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
>> ---
>>  mm/hugetlb.c | 26 ++++++++++++--------------
>>  1 file changed, 12 insertions(+), 14 deletions(-)
>> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index b8e844911b5a..9473eb6800e9 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1035,20 +1035,18 @@ static struct page *dequeue_huge_page_node_exact(struct hstate *h, int nid)
>>  {
>>  	struct page *page;
>>  
>> -	list_for_each_entry(page, &h->hugepage_freelists[nid], lru)
>> -		if (!PageHWPoison(page))
>> -			break;
>
>I don't see how it can reduce redundant check, just two different
>styles.
>

list_for_each_entry() stops by checking whether the item reach list head.

>> -	/*
>> -	 * if 'non-isolated free hugepage' not found on the list,
>> -	 * the allocation fails.
>
>But the above code comment seems stale, it checks HWPoision page()
>directly, but not the old isolated page checking.
>
>> -	 */
>> -	if (&h->hugepage_freelists[nid] == &page->lru)
>> -		return NULL;

And here the check is done again, if we really iterate the whole list.

By take the code in the loop, we can avoid this check.

>> -	list_move(&page->lru, &h->hugepage_activelist);
>> -	set_page_refcounted(page);
>> -	h->free_huge_pages--;
>> -	h->free_huge_pages_node[nid]--;
>> -	return page;
>> +	list_for_each_entry(page, &h->hugepage_freelists[nid], lru) {
>> +		if (PageHWPoison(page))
>> +			continue;
>> +
>> +		list_move(&page->lru, &h->hugepage_activelist);
>> +		set_page_refcounted(page);
>> +		h->free_huge_pages--;
>> +		h->free_huge_pages_node[nid]--;
>> +		return page;
>> +	}
>> +
>> +	return NULL;
>>  }
>>  
>>  static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask, int nid,
>> -- 
>> 2.20.1 (Apple Git-117)
>> 
>> 

-- 
Wei Yang
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