linux-kernel - Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones

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Date:	Thu, 24 Nov 2011 10:07:55 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Michal Hocko <mhocko@...e.cz>,
	Christoph Hellwig <hch@...radead.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Shaohua Li <shaohua.li@...el.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 3/5] mm: try to distribute dirty pages fairly across
 zones



Can I make a question ?

On Wed, 23 Nov 2011 14:34:16 +0100
Johannes Weiner <hannes@...xchg.org> wrote:


> +		/*
> +		 * When allocating a page cache page for writing, we
> +		 * want to get it from a zone that is within its dirty
> +		 * limit, such that no single zone holds more than its
> +		 * proportional share of globally allowed dirty pages.
> +		 * The dirty limits take into account the zone's
> +		 * lowmem reserves and high watermark so that kswapd
> +		 * should be able to balance it without having to
> +		 * write pages from its LRU list.
> +		 *
> +		 * This may look like it could increase pressure on
> +		 * lower zones by failing allocations in higher zones
> +		 * before they are full.  But the pages that do spill
> +		 * over are limited as the lower zones are protected
> +		 * by this very same mechanism.  It should not become
> +		 * a practical burden to them.
> +		 *
> +		 * XXX: For now, allow allocations to potentially
> +		 * exceed the per-zone dirty limit in the slowpath
> +		 * (ALLOC_WMARK_LOW unset) before going into reclaim,
> +		 * which is important when on a NUMA setup the allowed
> +		 * zones are together not big enough to reach the
> +		 * global limit.  The proper fix for these situations
> +		 * will require awareness of zones in the
> +		 * dirty-throttling and the flusher threads.
> +		 */
> +		if ((alloc_flags & ALLOC_WMARK_LOW) &&
> +		    (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
> +			goto this_zone_full;
>  
>  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
>  		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {

This wil call 

                if (NUMA_BUILD)
                        zlc_mark_zone_full(zonelist, z);

And this zone will be marked as full. 

IIUC, zlc_clear_zones_full() is called only when direct reclaim ends.
So, if no one calls direct-reclaim, 'full' mark may never be cleared
even when number of dirty pages goes down to safe level ?
I'm sorry if this is alread discussed.

Thanks,
-Kame


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