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

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Message-Id: <20061204131959.bdeeee41.akpm@osdl.org>
Date:	Mon, 4 Dec 2006 13:19:59 -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 12:17:26 -0800 (PST)
Christoph Lameter <clameter@....com> wrote:

> > 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.
> 
> Mel's approach already mananges memory in a chunks of MAX_ORDER. It is 
> easy to just restrict the unmovable types of allocation to a section of 
> the zone.

What happens when we need to run reclaim against just a section of a zone?
Lumpy-reclaim could be used here; perhaps that's Mel's approach too?

We'd need new infrastructure to perform the
section-of-a-zone<->physical-memory-block mapping, and to track various
states of the section-of-a-zone.  This will be complex, and buggy.  It will
probably require the introduction of some sort of "sub-zone" structure.  At
which stage people would be justified in asking "why didn't you just use
zones - that's what they're for?"

> Then we should be doing some work to cut down the number of unmovable 
> allocations.

That's rather pointless.  A feature is either reliable or it is not.  We'll
never be able to make all kernel allocations reclaimable/moveable so we'll
never be reliable with this approach.  I don't see any alternative to the
never-allocate-kernel-objects-in-removeable-memory approach.  
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