linux-kernel - Re: [PATCH RFC] mm,vmscan: only evict file pages when we have plenty

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Date:	Wed, 31 Oct 2012 11:39:50 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	klamm@...dex-team.ru, akpm@...ux-foundation.org, mgorman@...e.de
Subject: Re: [PATCH RFC] mm,vmscan: only evict file pages when we have plenty

On Tue, Oct 30, 2012 at 02:42:04PM -0400, Rik van Riel wrote:
> If we have more inactive file pages than active file pages, we
> skip scanning the active file pages alltogether, with the idea
> that we do not want to evict the working set when there is
> plenty of streaming IO in the cache.
> 
> However, the code forgot to also skip scanning anonymous pages
> in that situation.  That lead to the curious situation of keeping
> the active file pages protected from being paged out when there
> are lots of inactive file pages, while still scanning and evicting
> anonymous pages.
> 
> This patch fixes that situation, by only evicting file pages
> when we have plenty of them and most are inactive.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> ---
>  mm/vmscan.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2624edc..1a53fbb 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1686,6 +1686,15 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  			fraction[1] = 0;
>  			denominator = 1;
>  			goto out;
> +		} else if (!inactive_file_is_low_global(zone)) {
> +			/*
> +			 * There is enough inactive page cache, do not
> +			 * reclaim anything from the working set right now.
> +			 */
> +			fraction[0] = 0;
> +			fraction[1] = 1;
> +			denominator = 1;
> +			goto out;

Is there a specific reason for making this exclusive to global
reclaim?  The "force scan anon when file is low" HAS to be specific to
global reclaim because swapping may not be allowed in memcg limit
reclaim, but not scanning anon when there is enough easy page cache is
a legitimate memcg limit reclaim thing to do as well.

I.e. could this check be moved just below the

	/* If we have no swap space, do not bother scanning anon pages. */
	if (!sc->may_swap || (nr_swap_pages <= 0)) {
		noswap = 1;
		fraction[0] = 0;
		fraction[1] = 1;
		denominator = 1;
		goto out;
	}

section?
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