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Subject: Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     hannes@cmpxchg.org, mhocko@suse.com, mgorman@techsingularity.net,
        kirill.shutemov@linux.intel.com, hughd@google.com,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1557447392-61607-1-git-send-email-yang.shi@linux.alibaba.com>
 <87y33fjbvr.fsf@yhuang-dev.intel.com>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <1fb73973-f409-1411-423b-c48895d3dde8@linux.alibaba.com>
Date:   Thu, 9 May 2019 19:25:20 -0700
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On 5/9/19 7:12 PM, Huang, Ying wrote:
> Yang Shi <yang.shi@linux.alibaba.com> writes:
>
>> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
>> swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
>> still gets inc'ed by one even though a whole THP (512 pages) gets
>> swapped out.
>>
>> This doesn't make too much sense to memory reclaim.  For example, direct
>> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
>> could fulfill it.  But, if nr_reclaimed is not increased correctly,
>> direct reclaim may just waste time to reclaim more pages,
>> SWAP_CLUSTER_MAX * 512 pages in worst case.
>>
>> This change may result in more reclaimed pages than scanned pages showed
>> by /proc/vmstat since scanning one head page would reclaim 512 base pages.
>>
>> Cc: "Huang, Ying" <ying.huang@intel.com>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Mel Gorman <mgorman@techsingularity.net>
>> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
>> Cc: Hugh Dickins <hughd@google.com>
>> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
>> ---
>> I'm not quite sure if it was the intended behavior or just omission. I tried
>> to dig into the review history, but didn't find any clue. I may miss some
>> discussion.
>>
>>   mm/vmscan.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index fd9de50..7e026ec 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1446,7 +1446,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>   
>>   		unlock_page(page);
>>   free_it:
>> -		nr_reclaimed++;
>> +		/*
>> +		 * THP may get swapped out in a whole, need account
>> +		 * all base pages.
>> +		 */
>> +		nr_reclaimed += (1 << compound_order(page));
>>   
>>   		/*
>>   		 * Is there need to periodically free_page_list? It would
> Good catch!  Thanks!
>
> How about to change this to
>
>
>          nr_reclaimed += hpage_nr_pages(page);

Either is fine to me. Is this faster than "1 << compound_order(page)"?

>
> Best Regards,
> Huang, Ying