Re: [v2 PATCH] mm: vmscan: correct nr_reclaimed for THP

[Michal Hocko <mhocko@kernel.org>]

On Sat 11-05-19 00:23:40, Yang Shi wrote:
> 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.

You are technically right here. This has been a known issue for a while.
I am wondering whether somebody actually noticed some misbehavior.
Swapping out is a rare event and you have to have a considerable number
of THPs to notice.

> 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.

This is quite nasty and confusing. I am worried that having those two
unsynced begs for subtle issues. Can we account THP as scanning 512 base
pages as well?

> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Reviewed-by: Shakeel Butt <shakeelb@google.com>
> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com>
> ---
> v2: Added Shakeel's Reviewed-by
>     Use hpage_nr_pages instead of compound_order per Huang Ying and William Kucharski
> 
>  mm/vmscan.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fd9de50..4226d6b 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 += hpage_nr_pages(page);
>  
>  		/*
>  		 * Is there need to periodically free_page_list? It would
> -- 
> 1.8.3.1
> 

-- 
Michal Hocko
SUSE Labs