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SubjectRe: [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@sgi.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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