linux-kernel - Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that may be migrated

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Date:	Tue, 5 Dec 2006 08:14:58 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Mel Gorman <mel@....ul.ie>
cc:	Andrew Morton <akpm@...l.org>, Andy Whitcroft <apw@...dowen.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add __GFP_MOVABLE for callers to flag allocations that
 may be migrated

On Mon, 4 Dec 2006, Mel Gorman wrote:

> 4. Offlining a DIMM
> 5. Offlining a Node
> 
> For Situation 4, a zone may be needed because MAX_ORDER_NR_PAGES would have
> to be set to too high for anti-frag to be effective. However, zones would
> have to be tuned at boot-time and that would be an annoying restriction. If
> DIMMs are being offlined for power reasons, it would be sufficient to be
> best-effort.

We are able to depopularize a portion of the pages in a MAX_ORDER chunk if 
the page structs pages on the borders of that portion are not stored on 
the DIMM. Set a flag in the page struct of those page struct pages 
straddling the border and free the page struct pages describing only
memory in the DIMM.

> Situation 5 requires that a hotpluggable node only allows __GFP_MOVABLE
> allocations in the zonelists. This would probably involving having one
> zone that only allowed __GFP_MOVABLE.

This is *node* hotplug and we already have a node/zone structure etc where 
we could set some option to require only movable allocations. Note that 
NUMA nodes have always had only a single effective zone. There are some 
exceptions on some architectures where we have additional DMA zones on the 
first or first two nodes but NUMA memory policies will *not* allow to 
exercise control over allocations from those zones.

> In other words, to properly address all situations, we may need anti-frag
> and zones, not one or the other.

I still do not see a need for additional zones.
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