Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP

[Shakeel Butt <shakeelb@google.com>]

From: Yang Shi <yang.shi@linux.alibaba.com>
Date: Thu, May 9, 2019 at 5:16 PM
To: <ying.huang@intel.com>, <hannes@cmpxchg.org>, <mhocko@suse.com>,
<mgorman@techsingularity.net>, <kirill.shutemov@linux.intel.com>,
<hughd@google.com>, <akpm@linux-foundation.org>
Cc: <yang.shi@linux.alibaba.com>, <linux-mm@kvack.org>,
<linux-kernel@vger.kernel.org>

> 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>

Nice find.

Reviewed-by: Shakeel Butt <shakeelb@google.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
> --
> 1.8.3.1
>