linux-kernel - Re: [PATCH 2/2] mm: forcely swapout when we are out of page cache

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Date:	Wed, 9 Jan 2013 16:26:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Sonny Rao <sonnyrao@...gle.com>,
	Bryan Freed <bfreed@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 2/2] mm: forcely swapout when we are out of page cache

On Wed,  9 Jan 2013 15:21:14 +0900
Minchan Kim <minchan@...nel.org> wrote:

> If laptop_mode is enable, VM try to avoid I/O for saving the power.
> But if there isn't reclaimable memory without I/O, we should do I/O
> for preventing unnecessary OOM kill although we sacrifices power.
> 
> One of example is that we are out of page cache. Remained one is
> only anonymous pages, for swapping out, we needs may_writepage = 1.
> 
> Reported-by: Luigi Semenzato <semenzato@...gle.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  mm/vmscan.c |    6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 439cc47..624c816 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1728,6 +1728,12 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
>  		free = zone_page_state(zone, NR_FREE_PAGES);
>  		if (unlikely(file + free <= high_wmark_pages(zone))) {
>  			scan_balance = SCAN_ANON;
> +			/*
> +			 * From now on, we have to swap out
> +			 * for peventing OOM kill although
> +			 * we sacrifice power consumption.
> +			 */
> +			sc->may_writepage = 1;
>  			goto out;
>  		}
>  	}

This is pretty ugly.  get_scan_count() is, as its name implies, an
idempotent function which inspects the state of things and returns a
result.  As such, it has no business going in and altering the state of
the scan_control.

We have code in both direct reclaim and in kswapd to set may_writepage
if vmscan is getting into trouble.  I don't see why adding another
instance is necessary if the existing instances are working correctly.



(Is it correct that __zone_reclaim() ignores laptop_mode?)


I have a feeling that laptop mode has bitrotted and these patches are
kinda hacking around as-yet-not-understood failures...
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