linux-kernel - Re: [PATCH] mm: vmscan: count only dirty pages as congested

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Date:	Wed, 15 Oct 2014 13:05:44 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jamie Liu <jamieliu@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>,
	Greg Thelen <gthelen@...gle.com>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmscan: count only dirty pages as congested

On Wed, 15 Oct 2014 12:58:35 -0700 Jamie Liu <jamieliu@...gle.com> wrote:

> shrink_page_list() counts all pages with a mapping, including clean
> pages, toward nr_congested if they're on a write-congested BDI.
> shrink_inactive_list() then sets ZONE_CONGESTED if nr_dirty ==
> nr_congested. Fix this apples-to-oranges comparison by only counting
> pages for nr_congested if they count for nr_dirty.
> 
> ...
>
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -875,7 +875,8 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  		 * end of the LRU a second time.
>  		 */
>  		mapping = page_mapping(page);
> -		if ((mapping && bdi_write_congested(mapping->backing_dev_info)) ||
> +		if (((dirty || writeback) && mapping &&
> +		     bdi_write_congested(mapping->backing_dev_info)) ||
>  		    (writeback && PageReclaim(page)))
>  			nr_congested++;

What are the observed runtime effects of this change?
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