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

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Date:	Mon, 4 Dec 2006 12:06:11 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Mel Gorman <mel@...net.ie>,
	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 11:41:42 -0800 (PST)
Christoph Lameter <clameter@....com> wrote:

> > That depends on how we do hot-unplug, if we do it.  I continue to suspect
> > that it'll be done via memory zones: effectively by resurrecting
> > GFP_HIGHMEM.  In which case there's little overlap with anti-frag.  (btw, I
> > have a suspicion that the most important application of memory hot-unplug
> > will be power management: destructively turning off DIMMs).
> 
> There are numerous other uses as well (besides DIMM and node unplug):
> 
> 1. Faulty DIMM isolation
> 2. Virtual memory managers can reduce memory without resorting to 
>    balloons.
> 3. Physical removal and exchange of memory while a system is running
>    (Likely necessary to complement hotplug cpu, cpus usually come
>    with memory).
> 
> The multi zone approach does not work with NUMA. NUMA only supports a 
> single zone for memory policy control etc.

Wot?  memory policies are a per-vma thing?

Plus NUMA of course supports more that a single zone.  Perhaps you meant
one zone per node.  If you did, that's a pretty dumb-sounding restriction
and I don't know where you got it from.

> Also multiple zones carry with 
> it a management overhead that is unnecessary for the MOVABLE/UNMOVABLE
> distinction.

I suspect you'll have to live with that.  I've yet to see a vaguely sane
proposal to otherwise prevent unreclaimable, unmoveable kernel allocations
from landing in a hot-unpluggable physical memory region.
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