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Date: Tue, 9 Mar 2010 09:50:15 -0600 (CST)
From: Christoph Lameter <cl@linux-foundation.org>
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To: Mel Gorman <mel@csn.ul.ie>
cc: linux-mm@kvack.org, Nick Piggin <npiggin@suse.de>,
       Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>,
       Chris Mason <chris.mason@oracle.com>,
       Jens Axboe <jens.axboe@oracle.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] page-allocator: Under memory pressure, wait on
 pressure to relieve instead of congestion
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On Mon, 8 Mar 2010, Mel Gorman wrote:

> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 30fe668..72465c1 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -398,6 +398,9 @@ struct zone {
>  	unsigned long		wait_table_hash_nr_entries;
>  	unsigned long		wait_table_bits;
>
> +	/* queue for processes waiting for pressure to relieve */
> +	wait_queue_head_t	*pressure_wq;
> +
>  	/*

The waitqueue is in a zone? But allocation occurs by scanning a
list of possible zones.

> +long zonepressure_wait(struct zone *zone, unsigned int order, long timeout)

So zone specific.

>
> -		if (!page && gfp_mask & __GFP_NOFAIL)
> -			congestion_wait(BLK_RW_ASYNC, HZ/50);
> +		if (!page && gfp_mask & __GFP_NOFAIL) {
> +			/* If still failing, wait for pressure on zone to relieve */
> +			zonepressure_wait(preferred_zone, order, HZ/50);

The first zone is special therefore...

What happens if memory becomes available in another zone? Lets say we are
waiting on HIGHMEM and memory in ZONE_NORMAL becomes available?