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Date: Wed, 15 Jul 2009 19:48:20 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Rik van Riel <riel@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [PATCH -mm] throttle direct reclaim when too many pages are
 isolated already
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On Wed, 15 Jul 2009 22:38:53 -0400 Rik van Riel <riel@redhat.com> wrote:

> When way too many processes go into direct reclaim, it is possible
> for all of the pages to be taken off the LRU.  One result of this
> is that the next process in the page reclaim code thinks there are
> no reclaimable pages left and triggers an out of memory kill.
> 
> One solution to this problem is to never let so many processes into
> the page reclaim path that the entire LRU is emptied.  Limiting the
> system to only having half of each inactive list isolated for
> reclaim should be safe.
> 

Since when?  Linux page reclaim has a bilion machine years testing and
now stuff like this turns up.  Did we break it or is this a
never-before-discovered workload?

> ---
> This patch goes on top of Kosaki's "Account the number of isolated pages"
> patch series.
> 
>  mm/vmscan.c |   25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> Index: mmotm/mm/vmscan.c
> ===================================================================
> --- mmotm.orig/mm/vmscan.c	2009-07-08 21:37:01.000000000 -0400
> +++ mmotm/mm/vmscan.c	2009-07-08 21:39:02.000000000 -0400
> @@ -1035,6 +1035,27 @@ int isolate_lru_page(struct page *page)
>  }
>  
>  /*
> + * Are there way too many processes in the direct reclaim path already?
> + */
> +static int too_many_isolated(struct zone *zone, int file)
> +{
> +	unsigned long inactive, isolated;
> +
> +	if (current_is_kswapd())
> +		return 0;
> +
> +	if (file) {
> +		inactive = zone_page_state(zone, NR_INACTIVE_FILE);
> +		isolated = zone_page_state(zone, NR_ISOLATED_FILE);
> +	} else {
> +		inactive = zone_page_state(zone, NR_INACTIVE_ANON);
> +		isolated = zone_page_state(zone, NR_ISOLATED_ANON);
> +	}
> +
> +	return isolated > inactive;
> +}
> +
> +/*
>   * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
>   * of reclaimed pages
>   */
> @@ -1049,6 +1070,10 @@ static unsigned long shrink_inactive_lis
>  	struct zone_reclaim_stat *reclaim_stat = get_reclaim_stat(zone, sc);
>  	int lumpy_reclaim = 0;
>  
> +	while (unlikely(too_many_isolated(zone, file))) {
> +		schedule_timeout_interruptible(HZ/10);
> +	}

This (incorrectly-laid-out) code is a no-op if signal_pending().