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Date:   Fri, 10 May 2019 10:12:40 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     <hannes@...xchg.org>, <mhocko@...e.com>,
        <mgorman@...hsingularity.net>, <kirill.shutemov@...ux.intel.com>,
        <hughd@...gle.com>, <akpm@...ux-foundation.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP

Yang Shi <yang.shi@...ux.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@...el.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Mel Gorman <mgorman@...hsingularity.net>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.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);

Best Regards,
Huang, Ying

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